Morophon, the Boundless + Sparkcaster + Leyline of Mutation

Morophon, the Boundless

Sparkcaster

Leyline of Mutation

How it works

1. Cast Sparkcaster by paying 0.
2. When Sparkcaster enters, it triggers, returning itself from the battlefield to your hand.
3. Repeat.

This loop relies on Leyline of Mutation to bypass traditional mana costs, allowing you to cast spells by paying {W}{U}{B}{R}{G} instead. Morophon, the Boundless steps in to eliminate that entire colored cost by naming Sparkcaster's creature type, reducing its price to literally zero. Because Sparkcaster's own enter-the-battlefield trigger forces you to return a red or green creature you control to your hand, it can simply bounce itself back to your grip, ready to be cast for free over and over again.