Book Worm

Complete the Bestiary: every entry filled, which means killing (or for critters and NPCs, encountering) everything in the game, banner-grade rarities included. The Zoologist tracks your percentage and sells better goodies as it climbs. The tail is rare enemies (Nymph, Doctor Bones, Tim, the seasonal event cast) and every event fought to completion - so it naturally lands near the very end of a completionist run. Check the Bestiary's filter for empty silhouettes and hunt by biome. Genuinely one of the longest achievements in the game.