Bosses ‣ Scourge Guardians
Scourge Guardians will spawn at the beginning of a biome you're in if you stay there for 5 minutes. They can spawn in every biome, but they can't spawn in villages. The longer you stay, the more Scourge Guardians will spawn. They will follow you and try to hit you. They move similar to bees or bats. They are practically immortal, because (according to the bosses page) they can regenerate 100 HP per second.
Since Scourge Guardians spawn at the start of a dungeon, players usually have enough time to prepare/flee.