Naithan, Slumbering Giant

Naithan enters the battlefield with three
sleepyhead counters on it. As long as Naithan has
a sleepyhead counter on it, it can't attack or
block.
Whenever you cast a spell with mana value 5 or greater, remove a sleepyhead counter from
Naithan.
Naithan's power is equal to half of the highest
life total among all opponents.