SkunkyArt | WTFBOOOMSH — Spawn vs Demi-Fiend

They were both killed and soon revived by a devilish entity (Malebolgia and Lucifer respectively), who bonded them with a parasite that gave them all sorts of demonic and magical powers (K7-Leetha and the Magatama respectively).

Both would do their demonic life-savers' dirty work in the world they came from, though different from when they last were seen (Spawn returning five years after his death and Demi-Fiend's world being transformed into the inside-out Vortex World), and having their relationships with their loved ones strained by circumstance (Spawn's wife remarrying and having a child with her new husband and Demi-Fiend's friends becoming demons that sought to change the world, even if it meant getting through him).

Both would be involved in cosmic scale scuffles that would decide the faith of the entire world (Armageddon and the Conception respectively) and would be selected by a higher power (the Mother of Existence and Lucifer respectively) in order to stray from an existing conflict (God and Satan's battle and the clash of Reasons respectively).

This would bring out their full potential as something greater (the Mother of Existence using Spawn to prove that humanity is worth saving from God and Satan and Lucifer using Demi-Fiend to free the world of the Great Will's control and end the cycle of creation), with both boosting Spawn and Demi-Fiend's powers even further and moving them further and further away from being a mere human (Spawn merging with the Mother of Existence to become a god himself and Demi-Fiend inheriting more of Lucifer's power to become a full demon), turning them into god-slayers that even the forces of light and dark were terrified of.

Both also specialise in magic and demonic summons (Spawn's Legion and Demi-Fiend's demons respectively), and hail from stories that heavily incorporate real world religious tales and subtext into their worlds.