Text: If a champion takes damage, you may play this card. Attach Nightmare to that champion. At the start of each of their turns, the attached champion’s owner rolls a d6. 1-4: Take 1 damage. 5-6: Discard Nightmare.
Set: Evernight Vale
Artist: Kyle Herring
Because it is a response card, Nightmare can be played at any time and on any turn. But damage must be successfully taken. If a champion is targeted by a mercenary, but uses a Winged Guardian to redirect the damage, for example, a Nightmare could not be played.
Once attached, Nightmare cannot be removed unless card text removes it or the player rolls a 5 or a 6 at the start of their turn. Keep in mind that "at the start of each of their turns" means that this must be done before any cards are played. A player cannot heal their champion with a Healing Potion, for example, before they roll for the Nightmare.
If a champion dies because of a Nightmare, looting of the hand does not occur, because the champion died during its own player's turn.