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 Umbral Collar Zealot 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.
Resolve the Umbral Collar Zealot ability.
Repeat from step 3.
This loop relies on an ingenious self-sustaining recycling circuit enabled by Traveling Plague's death trigger. By using Umbral Collar Zealot to sacrifice the creature currently hosting the Aura, you send it to the graveyard just long enough for its recursion ability to target the fresh token you just made. Meanwhile, Archon of Sun's Grace acts as the engine's core producer, continuously churning out the necessary Pegasus targets for the Aura to latch onto every single time it re-enters the battlefield.