I decided to use "King" as the advancement name.
That advancement currently requires 22 different mob types from at least 7 different structures across the three vanilla dimensions. (If you don't find the "mountain" variant of ruined portals, you'll need to get the silverfish from a stronghold, but you still need a ruined portal for a zoglin.)
Loonium-spawned mobs are also now put into a unique team, which means they don't fight with each other anymore (important since ruined portals can spawn zoglins, which would attack every other mob), and that team name also helps identifying the mobs for the advancement.