Nim Deathmantle + Ashnod's Altar + Harried Spearguard

Nim Deathmantle

Ashnod's Altar

Harried Spearguard

How it works

1. Activate Ashnod's Altar by sacrificing Harried Spearguard, adding CC.
2. When Harried Spearguard dies, it and Nim Deathmantle triggers.
3. Resolve the Harried Spearguard trigger, creating a 1/1 Rat creature token.
4. Activate Ashnod's Altar by sacrificing the Rat, adding CC.
5. Resolve the Nim Deathmantle trigger from step 2, causing you to pay 4 to return Harried Spearguard from your graveyard to the battlefield, and attach Nim Deathmantle to it.
6. Repeat.

This loop turns a modest Goblin and a pair of artifacts into an infinite engine by neatly recycling its own costs. When you feed Harried Spearguard to Ashnod's Altar, it generates the exact four mana needed to pay for Nim Deathmantle's revival trigger, while also leaving behind a Rat token. That leftover token can then be sacrificed to the Altar to fuel the next iteration, letting you continuously loop the Spearguard back to the battlefield.