A Dungeon is a card type that exists outside the normal flow of play. It isn't cast from your hand or played as a spell; instead, it sits in the command zone. When an effect tells you to venture into the dungeon, you choose a Dungeon you own from outside the game and mark its first room, resolving that room's ability. Each further venture moves you to the next room. Once you complete the final room, that Dungeon is set aside, and you may choose a new one the next time you venture.
In a deck, Dungeons support strategies built around venturing, providing a series of incremental advantages as you progress. They can't be attacked, destroyed, or otherwise interacted with like permanents. Because they live outside your deck, they don't take up slots, but they only matter when your cards let you venture.
