Bosses are found in dungeons. To successfully complete a dungeon you will need to kill all bosses inside it. Bosses vary from dungeon to dungeon depending on the type, and what biome it is in. When killed a chest will spawn, which can be destroyed to obtain loot. When the final boss/the only boss is killed, a exit portal will appear which you can take by standing next to it, facing it and pressing the “E” key button. Exit portals will take you to the start of the dungeon.