A Chaos Spawn is a mindless mutant creature who once belonged to one of the intelligent mortal races of the galaxy and who has become so mutated through their servitude to Chaos that they devolved into a twisted and insane abomination.

Those seeking immortality and power through the ultimate attainment of Daemonhood in service to the Ruinous Powers face the constant possibility of being reduced to Chaos Spawn, either by accumulating too many mutations or being cast aside by their Chaos patrons as unworthy of further advancement because of their consistent failure to serve the will of the Chaos Gods.

More simply, they may lack the inherent qualities required for successful leadership of other Chaos servants and thus be of no further use to their patrons.

Chaos warband members who are transformed into Chaos Spawn will either be abandoned by their former comrades or kept as pets or as cannon fodder, to be herded into battle along with the rest of the Lost and the Damned.

Although physically powerful and extremely resilient, Chaos Spawn are insane, mewling creatures with no mental powers or self-awareness. Their former intelligence has been sacrificed to their failed ambition. Many a Chaos Champion of great promise has ended up as a seething mound of bone and blubber, monstrous and screaming with lunatic rage.

The Ruinous Powers are generous but erratic with their favours. When blessing one of their followers, a Dark God might make them stronger, tougher, faster, astonishingly beautiful, hyper-intelligent or otherwise improve their lot in the world.

Just as likely, the Chaos God may feel their loyal subject would benefit from their toes falling off, having an eye sprout in their navel or being transformed into a dribbling and imbecilic mountain of flesh.

Most Gifts of Chaos take the form of some physical mutation. The more of these gifts a Champion receives, the more potentially disastrous their cumulative effects can be -- even the body and genome of a Chaos Space Marine can only withstand so many mutations before he passes the point of no return.

Any Champion of Chaos, regardless of prestige, can find himself on the road to becoming a Chaos Spawn, for the gods are fickle. A Champion who does not earn himself the ultimate reward of daemonhood will surely become a Spawn unless he dies by some other means first.

Such is the fate of the Chaos champion -- those that survive are destined either for everlasting, diabolical glory or an ignominious end as a slavering heap of limbs and protrusions.

The outward appearance of a Chaos Spawn is utterly unpredictable and frequently changes over the course of a battle. No two are alike, and their make-up owes as much to the capricious energies of Chaos as it does to functionality.

A Spawn may have several sets of limbs, crab claws or tentacles, armour that bulges beneath the skin, tattered and useless wings, clumps of eye-stalks that wave like grass in a wind, a sinewy prehensile neck or a gaping maw of needle-thin teeth.

Some Spawn have insectoid bodies or heads, others resemble featureless quivering mounds of inverted muscle covered with spines that drip acidic pus. All, however, are hideous to behold and deadly to face in battle.

Source: https://warhammer40k.fandom.com/wiki/Chaos_Spawn