Old Everwyrm

Old Everwyrm enters the battlefield with
five crust counters on it.
Whenever Old Everwyrm becomes tapped,
remove a crust counter from it. At the beginning
of your end step, if there are no crust counters on
Old Everwyrm, transform it.
{t}: Add one mana of any color to your mana
pool. Use this only once per turn.