Crusader Kings III

To get this achievement, you must end the Iberian struggle by taking the "Detente" ending decision. You can track the requirements ingame at any time via the struggle menu.

You need to fulfill the following requirements:
- the current struggle phase must be "conciliation"
- be independant and be involved in the struggle (not just as an interloper)
- fully control at least one Iberian de jure kingdom
- have fame level 4 (exalted among men) or above
- have an alliance with every other independant involved ruler (interlopers and outsiders don't matter)
- DON'T control more than 50% of Iberia

Which starting character to pick?

If you start as any reasonably powerful involved character either in 867 or 1066 and don't have major mishaps during your early years, you should be able to end the struggle during the first conciliation phase. It can be done with any involved culture or religion, but it is significantly easier with a Muwalladi Muslim, because polygamy makes it easier to have many alliances (both by being able to produce more legitimate children and by marrying secondary spouses for the alliance yourself) and the Muwalladi as the more tolerant flavor of Islam have lesser opinion penalties with the other involved religions than Catholics and A'sharia have with each other. Having a culture with the egalitarian ethos (Asturleonese or Basque) can also be very helpful. Better don't start as a female ruler, because you'll need to have many children to get all those alliances, which is just way easier and less risky as a male ruler.

Getting to the conciliation phase of the struggle will take two or three decades. A young starting character might live to end the struggle themselves, but prepare your heir to do it just in case. The ruler to end the struggle should have decent values in diplomacy, so that he can sway and befriend foreign rulers to get them to accept marriage alliances. Good skills in intrigue can also come in handy, so that you can fabricate a hook on a reluctant ruler or make someone a widow(er) without getting caught, in case one of the other independant rulers just doesn't have any suitable marriage candidate who isn't already married.

I myself started as Alfonso of Leon (converting to Asturleonese culture) and narrowly missed ending the struggle as him (he died in 1094 when I already had the alliances and was just a few points of fame short of level 4) and ended up doing it as his heir, whom I had trained as a diplomat, in 1114.

Getting to the conciliation phase

The Iberian struggle always starts with the opportunity phase, then develops into either the hostility or the conciliation phase, whichever of the two gets to 1000 struggle catalyst points first. You can see a list of which events earn how many struggle catalyst points for which phase in the struggle menu. Simply put, involved rulers doing nice things to each other will earn points towards the conciliation phase and involved rulers doing bad things to each other will earn points towards the hostility phase.

If you don't actively work towards the conciliation phase as the player, the hostility phase will win the race almost every time, albeit not by a large margin. The best way to make sure we get the conciliation phase is to imprison a lot of involved ruler characters and then letting them go without demanding anything in return. This will earn you a whopping 25 points towards conciliation, while ransoming them would only earn you 5 points. The rank of the involved ruler doesn't matter, barons and mayors also count, including your own vassals, so do take the chance if you get to imprison some landed criminals or maybe even let a little civil war happen and then be merciful towards the rebelling rulers.
Befriending/seducing/romancing different faith rulers and getting different faith alliances also earns points towards conciliation, but it's way less and it can only be done so many times.

Preparing to get those alliances during the opportunity phase

- Expand your realm and try to eliminate or vasallize some of the competition first.

While you technically don't have to be very powerful to get the Detente ending (one Iberian kingdom is enough), being the most powerful ruler within Iberia makes you more desirable as an ally. Of course the less independant involved rulers are left, the easier it will be to get an alliance with all of them. You'll want to get most of the fighting done during the opportunity phase, because the struggle clash is less effective during the conciliation phase (expensive and only targets one county) and hostile actions during the conciliation phase cause progress towards the next phase. You need to control less than 50% of Iberia, but it doesn't hurt to get close to it. If you should conquer too much, you can always grant independace to non-de jure vassals later, ideally after first making them allies.

- improve acceptance with all involved cultures and religions.

Raise cultural acceptance by having counties of different cultures in your realm and giving at least some of them to vassals of their culture.

If possible, unlock the "desirable match" dynastie legacy (first one from the glory path), which gives you a bonus for marriage acceptance.

If you expect your current ruler to live long enough to end the struggle, start befriending those rulers that you don't fight and try to get lifestyle perks that reduce the different culture and different faith mali to opinion. If you can spare enough fame, establish the xenophilic cultural tradition.

If you aren't already Mozarabic or Muwalladi, consider converting to one of those faiths, since they have a less harsh different faith opinion penalty with everyone else than Catholic and A'sharia have with each other.

- breed like a rabbit

To get many alliances, you'll need many children. As a Muslim, just get your maximum number of wives and seduce all of them. As a Christian, consider having a few affairs with unmarried women and don't keep it secret, so you can claim the bastard children as your own. You don't have to legitimize them to use them for marriage alliances, but they need to be unambigously yours. The adulterer traits and loss of devotion is a downer, but it's better than ending up with not enough family members to conclude the alliances. You can't lose another level of devotion when you're already a sinner (level 0) and religious wars against other involved rulers are banned anyway during most of the struggle phases, so piety points are less important than they usually are.

- have as few allies as possible

This might seem counterintuitive at first, because your end goal is to have an alliance with every involved ruler, so shouldn't you go for every alliance you can get? NO! You can't take the detente ending before the conciliation phase and much can happen in 20 years. Independant rulers die, get vasallized or dethroned. Marriages get divorced. Often times, you won't be able to get your child back to conclude a new alliance. If you married a daughter off to some second son of another independant ruler, chances are your allie dies or gets dethroned, but your daughter is still married and now only gives you a borderline useless alliance with a vassal count that still counts towards your number of alliances that lowers alliance acceptance chances for everyone else.

Thus you should only get two to three allies during the opportunity phase that are actually useful for you to help in your wars and that are likely to last. Save your additional children for when the conciliation phase starts and then get the alliances all at once.

For your few alliances during the opportunity phase, prefer alliances where you can marry your relative to the foreign ruler himself or their heir, so hopefully you can negotiate an alliance with the successor without having to arrange a new marriage. Another good strategy to get a helpful alliance without having to permanently "spend" a child for it is a pro forma betrothal of a baby daughter to a very old ruler, who'll likely die before the actual marriage would be concluded. This works especially well with older muslim rulers who already have a capable first spouse and enough kids, so being betrothed to a future second spouse they'll never actually see is not a problem for them.
Avoid having alliances with your vassal relatives, because every additional alliance will make it harder to get more.

- get the perk for the forced vassalisation casus belli (diplomacy skill tree)

A forced vassalisation war will often be the quickest and easiest way to take out an independant duke or count who for whatever reason just won't conclude an alliance with you, be it that he just hates you too much or simply doesn't have any suitable marriage candidates, so try to have that perk when the conciliation phase starts.

Concluding all the alliances and ending the struggle

Once you have reached the conciliation phase and fulfill all the other requirements (king, high enough level of fame), conclude alliances with all other rulers in quick succession. If you have any time-consuming preparations to make (like befriending someone who otherwise wouldn't accept or murdering someone), get that done before you conclude the alliances with the easy candidates. Remember that each new alliance lowers acceptance for further alliances, so start with the ones you want to negotiate without a marriage and then proceed with the ones who'd be most reluctant to agree, starting with the powerful ones of a different faith and saving the weak ones who share your faith for last.

Which other achievements to get in the same playthrough?

Unlike the status quo ending, the detente ending unlocks the creation of the Empire of Hispania and it does allow you to keep conquering and converting other parts of Iberia afterwards, it just permanently bans religious wars between the involved cultures, but you can still use mundane casus belli and you can still wage religious wars outside of Iberia. Thus the achievement can be combined with most Iberia achievements from the base game and the DLC that require you to conquer Iberia and/or convert all of Iberia to a certain faith. Becoming emperor of Hispania is actually quite easy after the detente ending if you were close to controlling 50% before, because you get a boost to involved culture and religion opinion and you are already allied with all the others, so you can easily persuade many of the independant neighbor dukes to swear allegiance to you.
Converting all of Iberia to your faith is easier with the dominance ending (which lets you choose a permanent bonus to conversion speed at the cost of a different faith opinion malus as a result of the decision), but you can get one of those three (Reconquista, Al-andalus, Andalusian Inquisition) done together with the detente ending instead of having three dominance ending playthroughs.

The big cultural acceptance boost in the end also gives you a major legs up for the "true tolerance" achievement (having 10 or more cultures in your realm with more than 90% acceptance).