See also the List of all unique cards and the unique category.

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
Unique Cards

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.

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