Crusader Kings III

For this achievement, you need to start in 867 as High Chieftain Siemowit Piast of Polania and lead his dynasty to rule the Kingdom of Poland (having the title is sufficient, you don't necessarily have to control all of its de jure land), convert your government form to feudalism and own regalia, crown, weapon and armor of the rarity famed or better (illustious, mythical). Those objectives can be fulfilled in any order, but you'll likely get the artifacts last, because famed artifacts are rare in the early game and you need to be feudal to get some of the things that improve your chances to produce famed artifacts. The artifacts need to function in your character inventory slots, famed crowns and weapons that only function as pedestal item court artifacts (like e.g. the crown of Justinian in the Byzantine Empire) don't count.

1) Conquering Poland

You start as an underdeveloped tribal nation, but the good news is your direct neighbours aren't any better off and your starting duchy is reasonably strong, so conquering and uniting tribal Poland is the easiest part of this achievement. You should have at least one good ally nearby (for me, Silesia worked well, we pretty much took turns starting wars and helping each other expand our territory and once I had enough to found Poland, they happily volunteered to become my vassal). Be careful not to move all of your troops too far away from your capital, because all of your tribal neighbors love to raid and a succesful raid on your capital can cost you valuable courtiers and even family members, so you should always keep some troops in reserve that are ready to drive a raiding army away.
The Polish culture has access to Konni, a special type of light cavalry with overall better stats and great pursuit. Best men at arms combo for the early game is several troups of Konni plus spearmen to counter enemy cavallery.

More difficult than initially conquering Poland is KEEPING it long enough to complete the rest of our objectives, since all of Poland is underdeveloped tribal territory with low income and there will likely soon be some more powerful and more advanced nations to the west and South (East Francia, Bavaria, Italy, Hungary). In the East, Russia and Galicia-Vohylna also have the potential to snowball into quite powerful (albeit tribal) kingdoms and you will also regulary be harassed by rogue norsemen armys of about 8000 seeking to conquer one of your duchies as a new home.

I therefore recommend not stopping at conquering Poland, but growing bigger, possibly aiming for the Empire of West Slavia, to become a nation strong enough to deter foreign invaders. Getting your weak western Slavic neighbours early (Bohemia, Moravia, Polabia) will also make it less likely that the HRE gets formed.
You should also try to always have at least one or two strong allies.

2) Becoming feudal

Converting your realm from tribal to feudal is a major decision and requires the following:
- have absolute tribal authority as an independant ruler
- have at least development 5 in your capital
- have an organized religion
- have 70% of the non-local tribal age innovations unlocked

However, don't rush becoming feudal the second you fulfill those prerequisites. Since all of your land is pretty underdeveloped, you will start out with a very low gold income. Turning feudal will destroy the few tribal buildings that gave additional income, will drastically reduce your levies and all of your men-at-arms will now require to be paid in gold instead of honor. Also you lose the ability to raid. Thus converting to feudalism prematurely can weaken you drastically and lead you into state bankruptcy. Make sure you have the technologies unlocked that are required to build holding improvements and also the city builders innovation to be able to fill your vast empty land with additional holdings and save up about 5000 gold, so you can immediately found some cities and temples in your domain and build improvements in your holdings that give you more income, so that you can sustain your maximum men-at-arms. Ideally, the monarch who introduces feudalism should have been in stewardship lifestyle to unlock the architect skill tree.

To reach development 5, simply have your steward increase development in your capital. Your starting capital county Poznan is quite good (5 holdings and has a farmland holding), but Poland's de jure capital Krakow is better in the long run (6 holdings, one of them farmland, and a building spot for a university. You'll likely be done with the achievement though at about the time you reach the 30 development required to build the university, so feel free to stay in Poznan if you don't want to play a long time Poland campaign).

To get an organized religion, you could either convert to catholicism or orthodoxy or you could try to capture 3 Sloviansk holy sites and reform Sloviansk paganism. Staying pagan is by far the most difficult option, since it makes you a prime target for religious wars from your powerful catholic neighbors to the west and you'll have to rely on the more unstable tribal pagan nations to the east for alliances. You'll also have to conquer quite a lot of land to get to your third holy site. By far your easiest option is to convert to catholicism. You can play nice with your western neighbors, earn a lot of piety by fighting the pagans to the east and regularly use some of that piety to ask the pope for gold. Going orthodox gives you a better bonus for research progress, but makes getting alliances with catholics more difficult and IMHO the option to ask the pope for gold is more valuable than the progress bonus. I'd recommend to stay pagan while you conquer Poland, then convert once you own most of the land you need and have a good standing with your vassals, so that many of your realm's counties will convert alongside you.

Getting 70% of the tribal innovations will take you some time, since you start out with only 5 out of 14 innovations and and average development of 1. Also the Polish culture has the Stauch Traditionalists tradition, which decreases your cultural fascination bonus and makes it more difficult to create hybrid cultures. Definitely breed your heirs for intelligence and try to get an heir with learning education.
Conquering the Prussians and forming a hybrid culture with them can speed up research considerably, since you instantly get their research progress, can adopt their buerocratic base ethos and get rid of the staunch traditionalists tradition in the process. You'll need to push cultural acceptance up to 80% instead of the usual 40%, but it's still worth it.
If you are fast with your conquests, taking the "unite the West-Slavs" decision can also help you by instantly unlocking two innovations.

3) Getting famed artefacts

There are essentially 3 ways to get those 4 famed artefacts.

a) Making them yourself

You can make an artefact by either taking the "comission artefact" decision to produce a specific artefact or by sponsoring the inspiration of an artisan who shows up randomly at your court. The latter often have better skills and thus a far better chance to actually produce a famed artefact, but they show up at random and you can't pick which kind of artefact they will produce. The base chance of a artefact created by the comission decision to be famed or better is zero, but you can actually get a good chance to produce a famed artifact even by that decision if you stack some modifiers:

- raising the quality of your lodgings in your court anemities to maximum (5) (can only be done after converting to feudalism, since tribal courts are stuck with a maximum court anemity level of 2)

- have a crown smithies duchy building in your capital (requires the early medieval innovation burhs to build)

- have a mine in your personal domain. The county of Caslaw in Bohemia has a building slot for a mine.

With those three, you should be able to produce 4 famed artefacts pretty quickly, although there still is a chance that artefacts turn out to be only masterwork. Even with only two of those you already have a decent chance for a famed item.

b) Stealing them

Your chance to loot a famed inventory artefact at random while raiding or sieging other lord's capitals are pretty low, since only court items and unequipped items can be obtained that way. You only have a chance to loot an equipped item if you kill the wearer in a battle.

You can steal specific artefacts though if you have claims on them. To get an artefact claim on a foreign artefact that doesn't actually have anything to do with your dynastie, you'll need the perk "accomplished forger" from the diplomat skill tree. You can then just browse any lord's inventory until you find a famed artefact and then forge a claim on it at the cost of a big chunk of fame and negative opinion of the target character.
Once you have the claim, you can get the artefact by doing one of the following:
- straight up demanding it from the owner (if the owner is military weaker and/or a bit of a coward, they might just hand it over)
- duell the owner for the artefact (owner can refuse if their prowess is significantly lower than yours)
- steal the artefact with a hostile intrigue scheme
- declare an artefact claim war on the owner

c) winning them in a tournament (requires tours and tournaments DLC)

Host a tournament yourself and set the price quality to the highest level, which guarantees the to be famed or illustrious. Martial competitions (melee, duell, wrestling, archery, joust) have a good chance for the price to be a weapon or armor (sometimes you only get a trinket or pedestal item though). If you host the tournament yourself, the duell contest is very easy to win provided you have decent enough prowess to qualify, because you can just claim that your opponent cheated and disqualify them if you lose (at the cost of some fame loss and tyranny points). This method can only get you weapon and armor though, for crown and regalia you'll have to use one of the other two methods.