Revanent of Nol Cathedral

Revanent of Nol Cathedral costs X less to cast, where X is the highest power among creatures
you control that died this turn.
First strike, unblockable.
"Eleven bells chime, in dark of night they call the time. Twelve bells call, long they ring to save us all. No bells call, no people at all. A silent toll at the Cathedral of Nol."