Coffin-Swap Necromancer

{6}{b}, {t}, Sac a creature: Destroy target creature
then return it to the battlefield under your
control. This ability costs {1} less to activate for
each creature card in your graveyard.
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"There is no naturally occurring power so strong
as that expelled when a being passes from the
realm of the living to that of the dead."
- Meta-magic lecture at the Academy