Souleaters seem to be formed only by Geb who are corrupted by the Blood and join the Blessed.

They outwardly resemble geb that have been horribly burned to death, and they glow with an inner fire. Their skin is blackened and flaking, showing wet red muscle beneath.  Inside their mouths is a burning hot white vortex of energy, which can actually project outward and snake out like a burning white-hot tentacle to constrict and stab its victims, burning them alive and drawing out their life force to be consumed.

The Blessed