There are currently seven bosses in the game: The Concierge, Conjunctivius, The Time Keeper, The Giant, The Hand of the King, a 5 BSC exclusive boss, and Mama Tick, which is only available with the Bad Seed DLC.
As the first boss, the player may find one of these three, depending on the path that they take.
The first boss encountered in the game. Requires Vine Rune. Can be reached either through the Ramparts or the Ossuary.
She is reached by going through the Toxic Sewers, then the Ancient Sewers, to the Insufferable Crypt.
The player must have absorbed the Vine Rune, Teleportation Rune, and Ram Rune in order to take this path. Alternatively at 3BSC+, it is possible to exit via the Insufferable Crypt from the Ramparts.
She can be reached if the player owns the Bad Seed DLC. It is only accessible by going through the Morass of the Banished and traveling to the Nest.
The fight can be skipped if the player sacrifices Mushroom Boi!.
Found in the Clock Room. Can be reached through either the Clock Tower or the Forgotten Sepulcher.
Unlocked by opening the gate to the Cavern in the Graveyard (requires Cavern Key).
Found in the Guardian's Haven. Can be reached through either the Cavern or the Forgotten Sepulcher (2BSC+ door).
Requires Rise of the Giant DLC.
The final boss of the game on 4 BSC and less. Found in the Throne Room. He is reached through High Peak Castle or Guardian's Haven.