Rule 309.2 · Dungeons
Dungeon cards begin outside the game. Dungeon cards aren’t part of a player’s deck or sideboard. They are brought into the game using the venture into the dungeon keyword action. See rule 701.49, “Venture into the Dungeon.”
Subrules
If a player ventures into the dungeon while they don’t own a dungeon card in the command zone, they choose a dungeon card they own from outside the game and put it into the command zone.
A dungeon card that’s brought into the game is put into the command zone until it leaves the game.
Dungeon cards are not permanents. They can’t be cast. Dungeon cards can’t leave the command zone except as they leave the game.
If an effect other than a venture into the dungeon keyword action would bring a dungeon card into the game from outside the game, it doesn’t; that card remains outside the game.