Bosses are aggressive, resilient enemies that offer a substantial challenge to players. Defeating a boss is usually instrumental in advancing the game in some way. Each has its own particular way of being summoned. For example, most bosses have associated summoning items that can be used to spawn them manually under certain conditions, while there are others that will appear after the player interacts in a certain way with the environment. Event bosses will only appear during their respective events and leave when the event ends. Mini-bosses also exist and appear only during special events, expressing the same amount of difficulty as many normal bosses.
Most bosses and mini-bosses can pass through blocks of all types, with the exception of King Slime, Lepus3DS Version, Turkor the Ungrateful's3DS Version body, the Flying DutchmanDesktop Version Console Version Mobile Version, and Golem's body.
For all non-event bosses, with the exception of the Lunatic Cultist, status messages will appear when that boss is about to spawn or has already spawned. Upon spawning, its particular theme music plays. During events, bosses and mini-bosses can appear multiple times and have no distinct spawn message. In the case of wave events like the Pumpkin and Frost Moons and the Old One's Army, status messages that appear at the start of each wave can indicate whether or not a certain boss or mini-boss will appear. Defeating a non-event boss concludes with a congratulatory message and several reward items.
