Athreos, Shroud-Veiled | Phyrexian Altar

Cards

You control at least one additional nontoken creature with a coin counter on it.

Activate
Phyrexian Altar by sacrificing the creature with a coin counter on it, adding one mana of any color.
When the creature dies,
Athreos's second ability and
Host of the Hereafter's ability trigger.
Resolve the
Host of the Hereafter trigger, putting a coin counter onto itself as well as +1/+1 counters if those were present.
Resolve the
Athreos trigger, returning the creature from your graveyard to the battlefield.
Activate
Phyrexian Altar by sacrificing
Host of the Hereafter, adding one mana of any color.
When
Host of the Hereafter dies, it and
Athreos's second ability trigger.
Resolve the
Host of the Hereafter trigger, putting a coin counter and +1/+1 counters onto the other nontoken creature.
Resolve the
Athreos trigger, returning
Host of the Hereafter from your graveyard to the battlefield.
Repeat.

This combo has been estimated to be Exhibition (Bracket 1+).