Cast Traveling Plague by paying 3, attaching it to another creature you control.
When Traveling Plague enters, Archon of Sun's Grace triggers, creating a creature token.
Activate Phyrexian Altar by sacrificing the creature Traveling Plague is attached to.
When the creature dies, Traveling Plague triggers, returning itself from your graveyard to the battlefield attached to the token created in step 2.
When Traveling Plague enters, Archon of Sun's Grace triggers, creating a creature token.
Repeat from step 3.
This clever loop relies on the recursive nature of Traveling Plague, which automatically returns itself to the battlefield whenever the creature it enchants leaves the battlefield. By sacrificing that enchanted creature to Phyrexian Altar, you generate a steady stream of any-color mana while sending the Aura straight to the graveyard to trigger its return. Every single time the Aura re-enters the battlefield, Archon of Sun's Grace spots the enchantment through its constellation ability and manufactures a fresh Pegasus token ready to host the Plague anew.