Kelleri, the Dark Storm

Indestructible
{x} is half your current life total.
Whenever you cast a sorcery spell, put a
+1/+1 counter on Kelleri, the Dark Storm.
At the beginning of your upkeep, if Kelleri, the
Dark Storms power equalls 20 or more, exile it
and destroy all creatures. For each creature you destroyed, create a 1/1 black zombie creature
token under your control.