Unique cards (denoted by a ◆ in front of the name) are cards that are so powerful that only one card is allowed to be in play at any given time. If a unique card ever becomes active, then any other card with the same title is immediately trashed. This cannot be prevented.
|Page 11, Column 2, Sidebar, Core Rule Book|
Some cards have a unique symbol (◆) in front of their title. There can be only one unique card of the same title active at a time. If a card with a unique title becomes active, any other card that shares its title is immediately trashed. This trashing cannot be prevented.