Urza, Lord High Artificer + Corridor Monitor + Retraction Helix + Starnheim Courser

Urza, Lord High Artificer

Corridor Monitor

Retraction Helix

Starnheim Courser

How it works

1. Cast Retraction Helix by paying , targeting a creature you control that doesn't have summoning sickness.
2. Cast Corridor Monitor by paying due to Starnheim Courser.
3. When Corridor Monitor enters, it triggers, untapping the creature targeted by Retraction Helix.
4. Activate Urza's first ability by tapping Corridor Monitor, adding .
5. Activate the creature targeted by Retraction Helix by tapping it, returning Corridor Monitor from the battlefield to your hand.
6. Repeat from step 2.