The El.

The El
The El is the collective name for the Dark star gods, each goes by a different name but the known ones are Crom Cruach, Grimnismal, The Patient One and their generals, Balor Birugderc and Moloch of the Teared Eye. These creatures are the gods of the Slyth. An alien insectoid race who feed the souls of mortals to the El. Their dimension is the El-World or Weird world and is separate from the cosmology of the multiverse.

The El themselves are said to have been born into the gaping maw of madness that is the far realm and from there unified under the rule of Grimnismal, the most powerful of the El. The El see most life as cattle waiting to be slaughtered and employ underhanded tactics to leave worlds ripe for the pickings. The El and Slyth are considered parasitic creatures and their corruption of worlds can leave the gods the chance with no choice but to destroy a world lest its corruption fester too greatly.

An El-Spawn is a term to describe a whole spectrum of mutated creatures warped into being by the corrupting influence of the EL corruption. Once the first mutation takes hold, the creature that was once now faces the inevitable dissolution of his mind and will, becoming something less than yet more than mortal. The fate that awaits nearly all aberrant mutants is to become a El- Spawn, a gibbering abomination existing only to serve the whims of its dark masters.

Some manage to stave off this doom for a time, committing great and terrible deeds to gather the Rewards and Gifts of the El Gods. But, for most, 9 out 10 times the fate of becoming a El-Spawn lies at the end of their dark and horrible road. The fate of a Spawn is to die, either on the field of battle by axe or sword, torn apart in the wilds by even more savage creatures, or literally ripped asunder by the corrupting energy that continues to course through its twisted body as it undergoes a terrible metamorphosis and a new Slyth is born. This is how the Slyth reproduce

Why mortals choose this is down to greed and self belief and the massive power the El will transfuse into his body. Many take the risk and many do not make it.