Prototype Portal + Dross Scorpion + Grinding Station + Su-Chi Cave Guard

Prototype Portal

Dross Scorpion

Grinding Station

Su-Chi Cave Guard

How it works

1. Activate Prototype Portal by paying 8 and tapping it, creating a token copy of Su-Chi Cave Guard.
2. When the token Su-Chi Cave Guard enters, Grinding Station triggers, untapping itself.
3. Activate Grinding Station by sacrificing Su-Chi Cave Guard and tapping it.
4. When Su-Chi Cave Guard dies, Su-Chi Cave Guard and Dross Scorpion trigger.
5. Resolve the Su-Chi Cave Guard trigger, adding eight C.
6. Resolve the Dross Scorpion trigger, untapping Prototype Portal.
7. Resolve the Grinding Station ability, causing target player to mill three cards.
8. Repeat.