When a player plays the Mictlantecuhtli (meek-tlahn-teh-COO-tlee) Skull Wall Card, the player can place a Skull Wall Barrier on the Yaotitlan any where they could play an active Warrior Card. That Serpent Stone is then unusable for the rest of the round.
After each player is left holding five cards to start, the players can choose to place one Skull Wall on any Serpent Stone that is not a Temple Stone or Keystone. The first player places the first Skull Wall; the second player places the second.
Players can only place one Skull Wall at the start of play. If a player chooses not to place a Skull Wall, that Skull Wall will not be used for the duration of the round.
If a player places a Skull Wall after playing a Mictlantecuhtli Card, the card is removed from the deck for that round. (In the event of a round where the discard deck must be reshuffled, a used Mictlantecuhtli Card cannot be included in the deck reshuffle.)