Teren, Damned Archmage

When an instant or sorcery card enters your
graveyard, you may put a -1/-1 counter on
Teren, Damned Archmage.
{1}{x}, X cannot be 0. remove X counters from
Teren, Damned Archmage: target instant or
sorcery gains flashback until end of turn.
The flashback cost is equal to its normal mana
cost minus X