Prototype Portal + Clock of Omens + Battered Golem + Darksteel Citadel

Prototype Portal

Clock of Omens

Battered Golem

Darksteel Citadel

How it works

1. Activate Prototype Portal by paying 0, creating a token copy of Darksteel Citadel.
2. When the Darksteel Citadel token enters the battlefield, Battered Golem triggers, untapping itself.
3. Activate Clock of Omens by tapping Battered Golem and the Darksteel Citadel token, untapping Prototype Portal.
4. Repeat.

This clever loop turns the indestructible land Darksteel Citadel into an endless manufacturing plant using Prototype Portal to repeatedly imprint and copy it for zero mana. Every time a new Citadel token arrives, Battered Golem's triggered ability wakes it right back up to help Clock of Omens tap away both the newly spawned land and the Golem itself. That clever pairing untaps your Prototype Portal so you can instantly repeat the cycle, yielding infinite tapped copies of your chosen land and endless triggers for any permanent that cares about artifacts entering the battlefield.