Hel, the Life-Giver

Whenever another creature enters the battlefield,
you gain 1 life and you put the top card of your
library into the graveyard.
At the begging of each end step, if a creature
entered the battlefield this turn, transform
Hel, the Life-Giver
If Hel, the Life-Giver would die, exile it instead,
and return it to the battlefield at the beginning
of your next upkeep.