Mortarion, the Pale King

At the beginning of your upkeep, put a -1/-1
counter on each creature you don't control.
At the beginning of your end step, put a -1/-1
counter on each creature you don't control.
Then, double the number of -1/-1 counters on
all creatures.
Whenever 5 creatures have died due to -1/-1
counters reducing their toughness to 0,
transform Mortarion, the Pale King.