Prototype Portal + Dross Scorpion + Yahenni, Undying Partisan + Cathodion

Prototype Portal

Dross Scorpion

Yahenni, Undying Partisan

Cathodion

How it works

1. Activate Prototype Portal by paying 3 and tapping it, creating a token copy of Cathodion.
2. Activate Yahenni by sacrificing Cathodion.
3. When Cathodion dies, Cathodion and Dross Scorpion trigger.
4. Resolve the Cathodion trigger, adding CCC.
5. Resolve the Dross Scorpion trigger, untapping Prototype Portal.
6. Resolve the Yahenni ability, giving it indestructible until end of turn.
7. Repeat.