The Creature with Bloodthirst does not have a -1/-1 counter on it.
Steps
Activate
Cauldron of Souls by tapping it, giving the Creature with Bloodthirst persist until end of turn.
Activate
Phyrexian Altar by sacrificing the Creature with Bloodthirst, adding one mana of any color.
When the creature dies, its persist ability and
Dross Scorpion trigger.
Resolve 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.
Resolve the
Dross Scorpion trigger, untapping
Cauldron of Souls.
Repeat.
Results
Infinite LTB.
Infinite ETB.
Infinite colored mana.
Infinite sacrifice triggers.
Infinite death triggers.
Metadata
In 7 decks according to EDHREC.
Bracket Tag
This combo has been estimated to be Exhibition (Bracket 1+).