Minecraft wiki entry for Achievement_guide, associated text: There are 2 ways to find pillager captains, from pillager patrols or from pillager outposts. Pillager outposts can be found in biomes where villages can generate. Pillager captains can be easily distinguished between other pillagers as this pillager will be wearing a ominous banner on their head. Kill this mob and you will get achievement, plus the Bad Omen effect.