The Illusion card summons an illusion of the player to fight alongside them, much like Mara's ability. The illusion is armed identically to the player. Higher card power increases the duration of the illusion.
The summoned illusion will be hostile to followers of Trog.
Prior to 0.16, decks needed to be wielded to be evoked, so this card prompted the player to choose a weapon for the illusion to wield.