When Siege-Gang Commander enters the battlefield, it and Mana Echoes trigger.
Resolve the Siege-Gang Commander trigger, creating three 1/1 Goblin creature tokens.
When the Goblin tokens enter the battlefield, Mana Echoes triggers three times.
Resolve all four Mana Echoes triggers, adding at least 16.
Activate Urn of Godfire four times by paying 8, adding .
Activate Siege-Gang Commander by paying C and sacrificing itself.
Sigil of the New Dawn triggers, causing you to pay C to return Siege-Gang Commander from your graveyard to your hand.
Resolve the Siege-Gang Commander activated ability, dealing 2 damage to any target.
Repeat.
This explosive loop starts when you cast Siege-Gang Commander, letting Mana Echoes scale up its mana generation exponentially as the three Goblin tokens hit the battlefield alongside it. Urn of Godfire steps in as a clever filter, converting that raw colorless energy into the precise colored mana needed to keep the engine humming. Finally, you use Siege-Gang Commander's own sacrifice ability to deal damage while feeding its remains directly into Sigil of the New Dawn, allowing you to pay the return cost and loop the whole process infinitely.