Crusader Kings III

1) What you need to do

To get this achievement, you need to take the "status quo" ending decision as a character involved in the Iberian struggle to end the struggle. Taking this decision is only possible in the compromise phase of the struggle, which is the third phase. The struggle begins with the opportunity phase at the start of the game, then transitions either into the hostility or conciliation phase, depending on which struggle catalysts are triggered more during the opportunity phase, and finally transitions into the compromise phase. If no one takes the status quo decision during that phase, it will eventually transition into the opportunity phase again and the cycle will begin anew. Each phase usually takes a few decades, so be prepared for a medium length campaign.

To be able to take this decision in the compromise phase, the following conditions must be met (you can look those up at any time in the ingame struggle menu under "ending decision"):

1. you must be involved (i.e. you must have both an involved culture and an involved faith and you must have your realm capital in Iberia)

2. you must be an independant ruler and fully control at least one of five specific Iberian duchies (Either Galicia, Toledo, Cordoba, Valencia or Barcelona)

3. You must either be at least at fame level 4 (exalted among men) or hold any Iberian kingdom title

4. No one may hold more than 50% of Iberia (that includes yourself!)

5. There must be no ongoing war between any independant involved rulers (wars between or against vassals and wars against interlopers or outsiders don't matter)

6. One of these three must be true:
a) all involved independant rulers have at least +60 opinion of you
b) more than 25% of Iberia is controlled by interlopers or uninvolved rulers
c) no other involved except you is a king or emperor

You can forget about option c, because the Iberian kingdoms are small and easy to create, so that one is very unlikely to happen and you can hardly actively push for it to happen when you aren't allowed to rule more than 50% of Iberia yourself (I guess you could conquer all of Iberia and then grant independance to all dukes outside of your de jure kingdom and take the decision immidiately after that, but that would be both weird and needlessly complicated ;-) ).

So there are basically 2 ways to solve this: Either raise your heir to become a great diplomat who'll make everybody love him (see chapter 2) or be a traitor and help interlopers reach the 25 % threshold (see chapter 3)

2) The diplomatic solution

Which starting character to pick?

It doesn't matter much which character you pick to start, since the one who will end the struggle will most likely not be your starting character, but their son or grandson, and you can always convert culture or religion later. The first step after the start of the game will be to establish an independant realm that is powerful enough not to be overrun by others, so it'll be easier if you pick someone who doesn't start too weak and isn't too bad at warfare. For raising cultural acceptance with everyone, it is useful to start as a culture who has the egalitarian basic ethos (or to convert to such a culture or form a hybrid culture with them as an already involved character). Among the Iberian cultures, the Basque and the Asturleonese have that ethos.
You don't necessarily have to start as an involved character, in fact you might want to start as a character from outside Iberia to get the "holidaying in Iberia" achievement in the same run. Duke Antso of Gascogne is a good choice for that, since all he has to do to get involved is to conquer a county from his weak neighbor Navarra and move his capital there.

It might be slightly easier to start in 1066 than in 867, because the advanced technology makes it easier to get a decent gold income out of a fairly small realm and the Islamic rulers have already split up into smaller kingdoms and duchies. If you start in 867, you must make sure that Al-Andalus breaks apart, either by dissolving it from within by a succesful dissolution rebellion, or by destabilizing it by murdering their rulers often enough to make it fall apart.

Establishing a stable realm

We don't need to conquer much. In fact, we shouldn't conquer too much in Iberia, because we need to control less than 50%. We just want a solid domain that gives us a good income and enough troops to not be easy prey.

In theory, we don't even need to be a king and only need to control one of the 5 duchies mentioned in the prerequisites. It is better to be a king though, since that gives you more diplomatic leverage (easier to get good marriage alliances, easier to befried foreign rulers). Also holding court can trigger court events to improve acceptance with other cultures (only if you have the royal court DLC, but you probably have that, because seriously, who buys the fate of Iberia DLC seperately?).

So get some allies and expand your realm moderately. If you want to make absolutely sure that no NPC ruler can end the struggle in the hostility phase via the dominance decision (which is unlikely, but it can happen, I have seen NPCs end the struggle), aim at controlling about 25% of Iberia.
The special rules of the Iberian struggle prevent religious wars against other involved characters, but we can use the convenient struggle clash casus belly, which lets us seize all counties of our opponent that directly border our realm during the opportunity or the hostility phase. If we need to expand our personal domain, we can still revoke titles from vassals of a hostile faith or fabricate claims as usual. Alternatively, if you are a vassal of a kingdom, you can try to coup your liege, in any case you need to get independant. If you are one of the Jimena family, you can also try to expand your realm via murder and inheritance. The usual stuff, you know the drill.
No need to worry too much about foreign ruler's opinion just yet, you can't conquer land without making a few people hate you and those grudges will wear off over time. Just maybe restrain yourself a little with the heretic burning and don't make an influential dynastie your arch rivals.

It is useful to incorporate counties from various different Iberian cultures into your realm and to take measures to raise the cultural acceptance for them (like giving some of those counties to vassals of the same culture, have your steward promote cultural acceptance and travelling around with a cultural emissary or strategically converting the culture of a county to cause another culture to directly border your culture), because a high cultural acceptance will also decrease the different culture opinion malus of foreign rulers from that culture.

Once you are satisfied with the size of your realm, stop conquering and start hoarding gold instead, so that your heir will have plenty of money for gifts and activities when the time comes. The "golden obligations" perk from the stewardship skill tree combined with some secret searching and blackmail in neighbor realms is a good way to make gold in an otherwise poor realm.

Breeding and educating an outstanding diplomat heir

Depending on whether you started as a young or an older character, the one to take the status quo decision will be either your child or your grandchild, so you need to prepare them for the task.

Try to breed for the congenital genius trait or at the very least the intelligent trait. Beauty is also useful for a diplomat, especially if your heir should happen to be female (since the attraction opinion bonus only works on characters who are attracted to your gender and most foreign rulers will be heterosexual men), but when you have to pick between the two, genius is way better than beautiful.

Try to unlock the early medieval divine right innovation, so you can raise your crown authority to absolute and freely pick your heir among your children. If you can't manage that in time, you'll need to employ other methods to make sure your genius diplomat becomes your heir, like disinheriting older siblings as the dynastie head, getting them to be monks or having them serve as knights in losing battles and hoping for a tragic outcome.

Ideally, you want a diplomat with a very high diplomacy skill and also a decent learning skill, because you'll use perks from both skill trees and diplomacy of course is the most important attribute for making people like you. If none of your kids has decent talent for a diplomacy education, it can also be done by a genius with the learning education.

If you can amass enough dynastie fame in time, unlock the "studious youth" dynasty legacy for a better chance at a good education trait for your heir.

If you aren't too bad in diplomacy, educate your heir yourself to make sure he gets favorable character traits. If you do that, make sure to pick up the pedagogy perk (it is a starter perk of the learning education, so you don't necessarily have to do the learning lifestyle for that, you can just do a bit of travelling and pick up 1000 XP in the learning lifestyle by visiting the right points of interest). If you are bad at diplomacy, don't educate the heir yourself and rather pick the best diplomate in your realm as their warden. Their actual skillpoints in diplomacy matter much more than their education focus, so if you have the choice e.g. between a diplomat with 14 diplomacy or a steward who just happens to also have 20 diplomacy, pick the steward. Also have a look at their character traits, because your heir is quite likely to get character traits of their guardian.

Avoid nasty character traits that are disliked by many (like sadistic, arrogant, callous, deceitful, paranoid) and character traits that are regarded as sinful in any of the involved religions. Try to get character traits that are regarded as virtuous by those religions that are hostile to his (so e.g. if your heir is Christian, try to get generous or just for an opinion bonus with Islamic rulers, if your heir is Islamic, try to get compassionate or honest for an opinion bonus with Christian rulers.

Also pick up the groomed to rule perk (diplomacy skill tree) for yourself to give your child a few extra skill points. This one works retroactively even if your child is already an adult and works regardless wether you educate the child yourself or not.

If you educate your heir yourself, go on a long pilgrim journy during that time. Your underage ward will tag along and earn the traveller and the pilgrim trait. If you have a court tutor (requires royal court DLC), have your heir learn as many of the languages that are spoken by other Iberian rulers as possible, since that halves the foreign culture opinion malus.

When your heir is close to adulthood, give him a secure county to rule. He'll earn lifestyle experience faster as a ruler and maybe even get additional skill points, because NPC rulers get random ruler events just like you get them as a player. Marry them to a spouse who also has a good diplomacy score.

Playing as the diplomat heir

When you take over, you are most likely in the second phase, not yet in the deciding compromise phase. Which is good, because that way you can prepare and take the ending decision, as soon as the compromise phase starts.

If you have the diplomat education, first pick the diplomacy lifestyle with foreign affairs focus (which is most likely to produce random events that improve opinion with foreign rulers). Redistribute the already unlocked skill points if necessary to get everything that improves the power of your befriend scheme and gives you skill points from the right skill tree and get "thoughtful" and "defensive negotiations" from the left skill tree for a foreign ruler opinion bonus. If you don't already know all the languages of the other independent involved rulers, also pick up "adaptive traditions" from the left skill tree, which allows you to learn languages and have another personal schemes like befriend going on simultaneously.
Once you have that, switch to a learning lifestyle (best one is medicine, to also reduce your risks of dieing prematurely) and pick up pedagogy, openminded (+ 15 different culture opinion) and apostate (+15 different faith opinion) from the scholar skill tree.

If you have the learning education, start with learning lifestyle instead and pick up everything from the scholar skill tree until you have "learn on the job" (which boosts your diplomacy greatly as long as you employ an excellent chancellor). Then switch to foreign affairs lifestyle and pick up the aforementioned diplomacy perks.

If you aren't either Mozarabic or Muwalladi, you should convert to either of those two faiths once your vassals like you enough to convert with you. Both of those religions are more tolerant than Catholicism and A'sharia muslims and thus get less harsh opinion mali with the other faiths.

In the meantime, put your excellent diplomatic skills to use and befriend as many foreign rulers as you can, sway or bribe the rest. Rising your prestige level is also good for general opinion, but avoid fighting offensive wars. Help out allies in their wars instead (the enemy rulers won't be miffed at all about that as long as you don't execute war prisoners or personally slay their close relatives in battle) or do peaceful activities that raise prestige, such as feasts or hunts. Invite neighbor rulers to those activities for additional opportunities to raise opinion and make friends (especially if you have the tours and tournaments DLC).

Do a worldly pilgrimage for a temporary opinion bonus with different faiths and cultures (I think that one also requires the tours and tournaments DLC, otherwise you are stuck with the standard pious pilgrimage - which isn't a bad idea for your reknown either, just not quite as strong for raising foreigner opinion).

If you did all that, getting the plus 60 opinion with all involved foreign rulers should be a breeze. Now all you have to do is to wait until the compromise phase starts and finally no one is at war with each other. If possible, get alliances with the weaker involved rulers to deter the stronger ones from attacking them. It may be a bit of a waiting game, but since the conditions during the compromise phase are tuned to encourage more peaceful behavior (easier alliances and interfaith marriage, higher casus belli cost, less effective struggle clash etc.), the time of universal peace in Iberia will eventually come. If there is one persistant warmonger who just won't stop attacking his neighbors, you might need to take action to get rid of them, but usually you can just wait it out.

When the conditions are met (a quill symbol will pop up to notify you that the decision is available), take the status quo decision at once (because you might not be the only ruler who can and will take it!) and the achievement is yours.

3) The traitor solution

For this one, we don't have to make anybody like us and instead help interlopers to control more than 25% of Iberia. Not my favorite solution, since it basically just turns this into another generic conquest campaign that you already have to do for so many other achievements. Also it seems a little cheesy to be like "Hey, fellow Iberians, look how powerful those interlopers have become, we really need to set our differences aside and unite against them" when you just put the interlopers there yourself. But it works and it may be the easier solution, especially if you just happen to be unlucky with your heirs and don't get a decent diplomat in time or have them randomly die.

For starter characters, the more powerful we start, the easier it will be. Use the opportunity and the hostility phase to expand agressively and get more than one kingdom under your control. Make sure you aren't stuck with confederate partition, so you don't get your realm forcefully split into independant kingdoms on sucession.

It is unlikely that outside kingdoms expand agressively into Iberia on their own, since Iberia has basically only two outside neighbours. The Mahgreb states in the South usually get along well with Islamic Andalusia and have no ambition to conquer Iberia, France (West Francia, Aquitaine) to the North usually gets busy with feuds against the other Karlings, against England or with internal conflicts. Interlopers that may be present usually either convert to local culture and become fully involved sooner or later or they are dethroned and vanish.

Thus the easiest and most surefire way to get those 25 % interlopers is to install them as rulers yourselve and grant them independance (by either granting them your additional kingdom titles or granting them duchies outside your de jure and then use the "grant independance" interaction). All you need for that is courtiers of an outsider culture or religion to whom you can grant titles. If you are king, those tend to appear as guests in your court at random. Otherwise, we can just invite random wanderers to our court, marry our female courtiers to unlanded outsider characters without court positions or recruit a few war prisoners from wars outside Iberia. It's best to wait until the compromise phase and grant them the land only when all other conditions for the final decision are already met, because random foreigners made dukes or kings tend to get overthrown or convert to local culture.

4) Which other achievements can you get in the same campaign

If you start as an uninvolved character, you can neatly combine this with the "holidaying in Iberia" achievement. Unfortunately, all of the other big Iberia achievements (like Sibling Rivalry, Reconquista, Andalusian Inquisition, Al-Andalus) are either made impossible or much more difficult by taking the compromise decision and should better be combined with a campaign in which you go for a different ending of the conflict.

If you solve it the diplomatic way, you can easily pick up "Friendship is magic" (by becoming best friends with a ruler from a different faith and using the friendship hook on them) and "histories best friends" (by seducing your best friend - which of course requires that you pick someone as your best friend whom you can seduce) on the way.

If you have the tours and tournaments DLC, you can also pick up "The very best" by participating in a tournament early and then playing a lot of boardgames to max out the wit track of the hastiluder trait while you make friends and wait for the compromise phase.