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How it works
Prerequisites
An opponent was dealt damage this turn.
The creature with bloodthirst does not have a -1/-1 counter on it.
Creature with Bloodthirst
Steps
1Activate Cauldron of Souls by tapping it, giving the creature with bloodthirst persist until end of turn.
2Activate Ashnod's Altar by sacrificing the creature with bloodthirst, adding .
3When the creature dies, its persist ability and Dross Scorpion trigger.
4Resolve the persist trigger, returning the creature from your graveyard to the battlefield, which enters the battlefield with at least one +1/+1 counter on it due to its bloodthirst ability.
5Resolve the Dross Scorpion trigger, untapping Cauldron of Souls.