Munchkin 6: Double Dungeons - Rules of Play says:

he card game Munchkin is all about kicking down Doors, killing Monsters, and stealing their Treasure – but where are the dank walls of the subterranean labyrinth where our heroes engage in their murderous exploits? Munchkin 6: Double Dungeons brings those underground lairs to the world of Munchkin. Now you can enter the Dungeon of Elvish Excess, where everyone is an Elf. Or take a Portal to the Dungeon of Manga Wrangling, where tentacles are even more dangerous. Or power up in the Dungeon of Unexpected Epicness - although it's hardly "unexpected", given the name. Each of the 20 double-sized Dungeon cards in Demented Dungeons adds a new rule that affects the entire party, and maybe the monsters, too! Use the 16 Portal cards to jump from one Dungeon to the next in your search for gold and glory.