The Ice Dragon enters the game when the Wall is destroyed. It is not initially placed in a domain, but instead is placed in the space where the Wall was.
The Ice Dragon acts differently in its 2 phases (noted by each side of its card):
1. It protects the Dragon Glass found North of the Wall until all 3 Dragon Glass Tokens have been collected by the players.
2. Once all 3 Dragon Glass Tokens have been collected, it serves the Night King and provides him additional movement to attack castles after he Evokes Terror.