The Great Southern Plantkill

Post-mechs, Plantera's Bulbs (pink, glowing) grow in the Underground Jungle - smash one with a pickaxe and she wakes up angry. Carve a big arena around a bulb first (a couple of screens of open jungle with platform rows), keep moving through phase one's dashes, and mind the tentacle crown in phase two. Beating her calms the dungeon's new horrors, drops the Temple Key (Temple Raider), and graduates the world to the endgame.