Dungeon Master
Dungeon Masters appear to be large, hostile versions of 
Puzzle Key Ghosts holding a giant key. They can spawn with an orbiting ring of Key Ghosts, which come in the same three colors as standard Key Ghosts (Red, Blue, and Green). They will slowly float towards Isaac, and will use their large key to periodically bat a Key Ghost towards Isaac as a projectile which deals damage on contact, but can also have them home in towards any nearby 
Colored Locks, opening them if the Key Ghost hits a lock of a matching color.
They are intangible while orbited by Key Ghosts, but become vulnerable once they run out, proceeding to charge at Isaac while swiping at him with their giant key. They can also spawn without any Key Ghosts and will start off in this state.
Notes
- Dungeon Master's swipe, when they have no Key Ghosts, can open special
Gray Locks. - Some Dungeon Master variants can regenerate their Key Ghosts indefinitely and only die when the rest of the room is cleared. These can be identified by their blue eyes.
- Some enemies will have Dungeon Lock symbols over their head in rooms with a Dungeon Master. These enemies will typically be locked behind colored locks, and disappear if all Dungeon Masters are killed before they can be freed.
Trivia
- Dungeon Masters were unused in the Afterbirth+ release of the Fiend Folio MOD.
- A "dungeon master", also sometimes known as a "game master", is a term typically used in Dungeons & Dragons and other tabletop role-playing games to refer to the player that fulfills the functions as a storyteller, being in charge of describing details and aspects of the adventure and what the other playing participant's characters experience.
- Dungeon Masters appear to have tally marks on their forehead like their Puzzle Key Ghost brethren.
Gallery
Dungeon Master variant with blue eyes.
The key that Dungeon Master leaves behind upon being defeated.
Dungeon Master's Key Ghosts, referred to as Key Fiends.
