Minecraft

Just made the literal easiest, no BS guide! :

For obtaining this achievement and "Revaulting" I'd suggest using ChunkBase (https://www.chunkbase.com/) for your current seed to know where the closest Trial Chamber is in new chunks loaded within this update or within a new world entirely.

For this achievement you have to kill a Pillager with an ominous banner to get the ominous bottle item, you then use this to get the Trial Omen effect, which transforms Trial Chambers into Ominous Trial Chambers.

Enter one of the Trial Chambers with the Trial Omen effect active, If you open one of the Ominous Vaults with the Ominous Trial Key (which can be found in pots, chests or from spawners) you will unlock the achievement.