Ashnod's Altar + Furystoke Giant + Hardened Academic

Ashnod's Altar

Furystoke Giant

Hardened Academic

How it works

1. Activate Ashnod's Altar by sacrificing Furystoke Giant.
2. When Furystoke Giant dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
3. Hardened Academic triggers, putting a +1/+1 counter on Furystoke Giant.
4. Repeat.

This loop turns Ashnod's Altar into an infinite colorless mana engine by putting Furystoke Giant through a relentless cycle of sacrifice and resurrection. Each time the Giant dies, its persist mechanic brings it right back with a -1/-1 counter, but Hardened Academic cleverly steps in to cancel that drawback out by placing a +1/+1 counter on it whenever a card leaves your graveyard. Because state-based actions cause the opposing counters to annihilate each other, the Giant is left completely clean and ready to be sacrificed to the Altar all over again. While Furystoke Giant also brings along a tap ability that deals 2 damage to any target via its enter-the-battlefield trigger, this particular loop focuses purely on generating an endless stream of death, sacrifice, and mana.