Sensei's Divining Top + Cloud Key + Rook Turret

Sensei's Divining Top

Cloud Key

Rook Turret

How it works

1. Cast Sensei's Divining Top by paying 0 due to Cloud Key.
2. When Sensei's Divining Top enters, Rook Turret triggers.
3. Holding priority, activate Sensei's Divining Top by tapping it, causing you to draw a card and put Sensei's Divining Top on top of your library.
4. Resolve the Rook Turret trigger, causing you to draw Sensei's Divining Top from your library, and then discard a card other than Sensei's Divining Top.
5. Repeat

This clever loop relies on Cloud Key naming "artifact" to completely eliminate the mana cost of Sensei's Divining Top, allowing you to cast it for free over and over again. As the Top enters the battlefield, Rook Turret sees the new artifact and triggers its loot effect, but before that resolves, you tap Sensei's Divining Top to draw a card and place the artifact itself back on top of your library. When the Rook Turret trigger finally resolves, you draw Sensei's Divining Top right back into your hand while discarding an unwanted card, priming the engine to start all over again.