Kenrith, the Returned King + Phyrexian Altar + Training Grounds + Zirda, the Dawnwaker

Kenrith, the Returned King

Phyrexian Altar

Training Grounds

Zirda, the Dawnwaker

How it works

1. Activate Phyrexian Altar by sacrificing another nontoken creature, adding .
2. Activate Kenrith's fifth ability by paying due to Training Grounds and Zirda, putting the creature card onto the battlefield from your graveyard.
3. Repeat.

This clever recursion engine centers around Kenrith, the Returned King's black-mana activated ability to pull creatures directly from the graveyard. Normally a cost of {4}{B} would be quite steep, but Training Grounds and Zirda, the Dawnwaker team up to dramatically slash those activation costs down to a mere single black mana. Once Kenrith brings a creature back, you simply feed it into the jaws of Phyrexian Altar, which happily sacrifices the creature to generate the exact {B} mana needed to fuel Kenrith's resurrection loop all over again.