Fae Flayer

The Empress of Light wakes when you kill a Prismatic Lacewing - a rare fairy-moth that flutters about the surface Hallow at night after Plantera. Fight her at night and every attack is dodgeable-with-practice bullet-hell; touch her in daytime and everything one-shots you (that's the separate Terraprisma challenge, not this achievement - night is fine). Long runway, top-tier wings, and learn the sword-wall and sun-dance patterns. Her loot is dazzling and mostly worth the bruises.