Prototype Portal + Clock of Omens + Grinding Station + Darksteel Citadel

Prototype Portal

Clock of Omens

Grinding Station

Darksteel Citadel

How it works

1. Activate Prototype Portal by paying 0 and tapping it, creating a token copy of Darksteel Citadel.
2. The Darksteel Citadel token enters, triggering Grinding Station, untapping Grinding Station.
3. Activate Clock of Omens by tapping the Darksteel Citadel token and Grinding Station, untapping Prototype Portal.
4. Repeat.

This clever loop hinges on Prototype Portal safely cataloging the indestructible Darksteel Citadel to continuously churn out copies of the land for zero mana. As each new copy hits the battlefield, Grinding Station's triggered ability rewards the entry by untapping itself, keeping the mill engine primed for action. Meanwhile, Clock of Omens acts as the ultimate catalyst, tapping the newly minted land token along with Grinding Station to untap Prototype Portal so the cycle can begin anew.