Kalista, Spear of Vengeance

Eminence — Whenever another creature is dealt damage for the first time, if Kalista, Spear of Vengeance is in the command zone or on the battlefield, put a vengeance counter on it.
Kalista costs {x} less to cast, where X is the number of creatures with vengeance counters.
When Kalista enters the battlefield, each creature with a vengeance counter fights another target creature of its controller's choice.