Ulalek, Fused Atrocity + Strionic Resonator + Kozilek's Unsealing

Ulalek, Fused Atrocity

Strionic Resonator

Kozilek's Unsealing

How it works

1. Cast an Eldrazi from your hand by paying its mana cost.
2. Ulalek triggers, the Eldrazi cast triggers if it has a cast ability, and Kozilek's Unsealing's first ability triggers.
3. Holding priority, activate Strionic Resonator by paying 2 and tapping it, copying Ulalek's trigger.
4. Resolve the copy of Ulalek's trigger, causing you to pay CC to copy the Eldrazi spell, Ulalek's trigger, the Eldrazi cast trigger if it had one, and the Kozilek's Unsealing trigger.
5. Resolve the Eldrazi cast trigger, if applicable.
6. Resolve the copy of the Kozilek's Unsealing trigger, creating two 0/1 Eldrazi Spawn creature tokens.
7. Activate both Eldrazi Spawn creature tokens by sacrificing it, adding CC.
8. Resolve the copy of Ulalek's trigger from step 4, causing you to pay CC to copy the Eldrazi spell, Ulalek's trigger, the Eldrazi cast trigger if it had one, and the Kozilek's Unsealing trigger.
9. Repeat from step 5.

This intricate loop begins when you cast an Eldrazi, setting off a cascade of triggers including Kozilek's Unsealing and the commander itself, Ulalek, Fused Atrocity. By using Strionic Resonator to kick off the copying process, you create an initial copy of Ulalek, Fused Atrocity's trigger that lets you pay colorless mana to copy everything else on the stack—including the original spell and triggers. The resulting Eldrazi Spawn tokens generated by Kozilek's Unsealing can then be immediately sacrificed for the exact mana needed to pay for successive iterations of Ulalek, Fused Atrocity's copied ability, sustaining an unstoppable engine.