Kenrith, the Returned King + Phyrexian Altar + Myr Battlesphere

Kenrith, the Returned King

Phyrexian Altar

Myr Battlesphere

How it works

1. Activate Kenrith's fifth ability by paying 4, returning Myr Battlesphere to the battlefield from your graveyard.
2. When Myr Battlesphere enters the battlefield, it triggers, creating four 1/1 Myr artifact creature tokens.
3. Activate Phyrexian Altar five times by sacrificing Myr Battlesphere and the four Myr tokens, adding 4.
4. Repeat.

This loop relies on Kenrith, the Returned King to pull Myr Battlesphere straight out of your graveyard using the black mana generated by Phyrexian Altar. Every time the Battlesphere enters, it floods the board with four Myr tokens that feed right back into the Altar along with the Battlesphere itself to fully refund the reanimation cost. This creates a closed ecosystem of endless sacrifice, entering-the-battlefield, and leaving-the-battlefield triggers, all fueled by the very pieces that make the loop self-sustaining.