Mana Convocation

Mana Convocation enters the battlefield with
X charge counters on it.
At the beginning of each player's upkeep, that
player gains control of Mana Convocation.
{t}: Add {c} to your mana pool for each charge
counter on Mana Convocation.
{3}: Put a charge counter on Mana Convocation.
Only any player who does not control this card
may activate this ability.