Azkaban, Remote Prison

{t}: Add {c}
{t}: Exile target tapped creature for as long as
Azkaban is on the battlefield. Activate this
ability only if you control a creature with power
greater than target creature. If the combined
power of all creatures exiled with Azkaban and
the combined power of all creatures your
opponents control are ever each greater than
10, sacrifice Azkaban.