Dungeon cards are nontraditional Magic cards that begin outside the game. They are not part of a player's deck or sideboard.
2021-07-23 • wotc
Dungeon cards may be put into a player's command zone using the venture into the dungeon keyword action found on some cards from the Adventures in the Forgotten Realms set. If any other effect tries to bring a dungeon card into the game, it fails to do so.