You play a rule card at the beginning of your turn before you draw your cards. Each time a rule card is played, it alters the basic rules of the game. When another rule card is played, the previous one is discarded. No matter how many players are playing, only one rule card may be in the game. As soon as it is put into play, the rule card is displayed in front of all opponents.
It is also possible to discard rules through the use of some cards' special powers, such as Wish (46/FR) or Ellorelloran (93/UD).
In the deck construction, these special cards are considered free cards; they don't count as part of a deck, which means that actual deck sizes are 56, 76, or 111 cards. Dungeon cards were originally introduced in the Dungeons expansion set. They could grand players some special advantages. You could consider them personal rule cards, with only their owner's effect.
How Dungeon Cards Work
Dungeon cards are shuffled into the deck like normal rule cards. On your turn during phase 2, you can play your dungeon from your hand. In the same turn, you can play a realm, a holding, and a dungeon.
A dungeon card can only be removed in three different ways. One way is by casting a Wish spell. Dungeons are discarded if the Wish is successfully cast.
With Gib Kcir, a non-realm card can be placed in the Abyss. This works on dungeons as well.
Lastly, you can attack the dungeon to remove the card. The attacks are done during phase 4, not during the battle. There must, however, be a "path" to a dungeon in order for it to be attacked (from razed realms). A dungeon cannot be attacked if the realms surrounding it are unrazed. Upon successful razing, the dungeon card is discarded. After this, the attacker gets the spoils of the dungeon. If the defender defeats the attacker, he or she gets the spoils of the dungeon. Either way, combat ends.
Dungeon spoils work the same way as regular spoils. There are alternative rules to play dungeon cards. Check out Optional rules for more Strategy and Fun section.