Kenrith, the Returned King + Phyrexian Altar + Training Grounds + Biomancer's Familiar

Kenrith, the Returned King

Phyrexian Altar

Training Grounds

Biomancer's Familiar

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 Biomancer's Familiar, putting the creature card onto the battlefield from your graveyard.
3. Repeat.

This engine creates an endlessly repeating life cycle of sacrifice and resurrection, fueled by cost reducers that make Kenrith, the Returned King's reanimation ability cheap enough to sustain. With both Training Grounds and Biomancer's Familiar actively lowering activated ability costs, Kenrith's {4}{B} graveyard return drops to just {B}. You can then sacrifice that creature to Phyrexian Altar to generate the exact black mana needed to pay for the reduced activation cost, keeping the loop going indefinitely.