Bloodtithe Harvester

When Bloodtithe Harvester enters, create a Blood token. (It's an artifact with "{1}, {T}, Discard a card, Sacrifice this artifact: Draw a card.")
{T}, Sacrifice Bloodtithe Harvester: Target creature gets -X/-X until end of turn, where X is twice the number of Blood tokens you control. Activate only as a sorcery.

Mayhem Devil

Whenever a player sacrifices a permanent, Mayhem Devil deals 1 damage to any target.

Claim the Firstborn

Gain control of target creature with mana value 3 or less until end of turn. Untap that creature. It gains haste until end of turn.