Mikaeus, the Unhallowed + Spike Feeder + Ashnod's Altar

Mikaeus, the Unhallowed

Spike Feeder

Ashnod's Altar

How it works

1. Activate Ashnod's Altar by sacrificing Spike Feeder, adding CC.
2. Spike Feeder's undying ability triggers, returning itself to the battlefield with a +1/+1 counter on it.
3. Activate Spike Feeder's second ability three times by removing three +1/+1 counters from it, causing you to gain six life.
4. Repeat.

This loop is a masterclass in recycling through Mikaeus, the Unhallowed, whose static anthem grants your non-Human creatures the undying ability. When you sacrifice Spike Feeder to Ashnod's Altar for two colorless mana, its undying triggers to bring it right back to the battlefield with a +1/+1 counter. You can then use Spike Feeder's activated ability to strip that counter away for life, leaving it clean of counters so the next iteration can repeat the cycle endlessly.