Mortuary + Ravenous Intruder + Bolas's Citadel + Shield Sphere

Mortuary

Ravenous Intruder

Bolas's Citadel

Shield Sphere

How it works

1. Activate Ravenous Intruder by sacrificing Shield Sphere
2. When Shield Sphere dies, Mortuary triggers, putting Shield Sphere from your graveyard onto the top of your library.
3. Resolve the Ravenous Intruder ability.
4. Cast Shield Sphere from the top of your library by paying 0 life.
5. Repeat.

This loop creates an engine of endless sacrifices by leveraging Bolas's Citadel, which allows you to cast Shield Sphere directly from the top of your library for free since its mana value is zero. Once cast, you can repeatedly feed that same Shield Sphere to Ravenous Intruder, turning its activated ability into an infinitely large threat. Every time the artifact creature hits the graveyard, Mortuary intercepts it to place it right back on top of your library, seamlessly resetting the entire sequence for Bolas's Citadel to play it again.