Dragon Throne of Tarkir
Khans of Tarkir — rare
Legendary Artifact — Equipment
Dragon Throne of Tarkir rulings:
2017-07-14: If the equipped creature’s power is negative while resolving the activated ability it gains, X is considered to be 0. (This is a change from previous rulings.)
2014-09-20: The value of X is determined as the activated ability resolves. If the equipped creature isn’t on the battlefield at that time, use its power the last time it was on the battlefield to determine the value of X.