The Killing Curse

Convoke
When The Killing Curse enters the field, spend x amount of mana to empower the curse.
The opponent then must also spend the same
amount in mana and can also convoke but,
convoking will sacrifice their creatures.
If the opponent reaches the mana amount, their
creatures will repel the killing curse back at the
caster. If a player is hit with the killing curse, their
life drops to 0.