Erhenast, the Impassive

Hexproof
Erhenast, the Impassive's power and toughness are each equal to the number of legendary creatures you control.
Erhenast, the Impassive can't attack or block unless it has to.
Whenever a legendary creature you control becomes the target of a spell or ability controls, Erhenast, the Impassive attacks this turn if able if it's your turn, and blocks this turn if able if it's not your turn.