Machina Citadel

When one or more artifact creatures you control
are destroyed, if Machina Citadel is in your
graveyard, you may put Machina Citadel onto
the battlefield.
{0}: Destroy another target artifact creature you control. If a creature is destroyed this way,
destroy each creature you don't control with
power less than or equal to that creature's power.
Activate only once each turn.