Achievement suggestion

[umbral bay]

A Bit too Early: in the 1066 start date, play as the Hohenzollern dynasty (count of Zollern) and found the Kingdom of Germany.

[rich sky]

Why the kingdom of Germany ?

[quaint sage]

because the Hohenzollerns would later become the kings of Prussia, the German state that unified modern Germany in the 1860s, which is what OP is referencing with "a bit too early," but they trace their origins back to the Counts of Zollern, where the dynasty got their name.

I also don't think this is an entirely appropriate achievement. The idea of a unified nation didn't really exist during the medieval period. "Countries" were composed of a patchwork of noble realms, themselves patchworks of the lesser nobles under them, all under the king. Even then that's not entirely accurate, as the French domains of the Angevin Empire technically were not part of England, they were part of France, but it just so happened that their liege was both a French noble and simultaneously the king of England.

Putting all of that aside, by the standards of Medieval Europe, "Germany" is already "Unified" under the HRE, and the HRE only started to disintegrate later on in history because unlike its neighbors, it never centralized under the monarchy and instead retained that highly decentralized feudal organization, but that is more within the scope of EU4 and not CK3.

[rich sky]

If they become kings of Prussia ,it would make more sense that the achievement would be about holding the kingdom of Pomerania and about the northern crusades , in my opinion.

[quaint sage]

then the achievement wouldn't be "a bit too early" it'd be the northern crusades. not disagreeing, we certainly do need the northern crusades in the game, but that's not what OP is referencing or suggesting

[rich sky]

yeah i guess you can say Prussia is still part of the middle ages so it would be too early but then why the kingdom and not the empire of germany ? Also ,there is already an achievement in like with having ''Germany'' as your title so it would make it,in my opinion ,too similar.