Bosses are aggressive, resilient enemies that offer a substantial challenge to players. Defeating bosses 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, Lepus Turkor the Ungrateful’s body ,the Flying Dutchman 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.
On the Desktop version and Mobile version, bosses will appear on the minimap. On Mobile, bosses are represented by a white skull with red eyes, and on Desktop, they each have their own specific icon. Dungeon Guardians also appear on the mini map with their own icon on Desktop.
These are the first bosses the player can encounter. Usually, defeating one of these bosses will give the player items and materials necessary to progress in the game, eventually leading to Hardmode.
Hardmode Bosses are significantly harder upgrades of and new additions to previous bosses. Defeating them is usually instrumental in advancing the game in some way. Each has its own particular way of being summoned, and warning messages appear whenever one is about to appear. Defeating a boss concludes with a congratulatory message and several items.
During Hardmode, each night has a one in two chance of spawning one of the three Mechanical Bosses that have not yet been defeated in the current world. This only happens after removing at least one Altar from the world, so it may be advisable to refrain from doing so if the player is not ready for a challenging fight.