A typical example of a Chaos Spawn-OH-SWEET-JESUS-NOOOOOOGLARBLBLBLBL!!
The Gods blessed you too much, and now I shall end your misery! Such a fate does not await me! - Thorgar the Blooded One, on the you-know-whats.
The Chaos Spa-, ahem, the unit in question were, in 4th and 5th Editions, considered by many to be the worst unit in any Chaos army book and, quite possibly, the entire game of Warhammer 40k (the Space Pope coming in a close second). They were so bad, absolutely nobody takes them. Not even for flavor or joke games. It became a meme that to even utter their name causes a horrid mutation turning you into a Chaos Spawn yourself. Wait! No! I didn't mean it- *OHSWEETJESUSMOTHEROFGODNOOOOOOOOOOO!!GLARBLBLBLBL*
Ahem... Continuing where my predecessor left off, you-know-whats should only be referred to as you-know-whats. That or they-who-shall-not-be-named.
In fluff, aspiring champions of chaos normally devolve into said units for any of the following reasons: if they use chaos solely for their own gain, their patron god thinks they're not worthy of their favor or have fallen out of their favor, they can't handle the sheer amounts of mutations they go through during their ascension into a full-fledged champion, or, you know, it's Tuesday and Tzeentch is feeling bored since there's not a lot of "Just as Planned" going around. When this does happen, Chaos Spawns devolve into no more than mindless heaps of mutated flesh who attacks anyone near them and eventually die after a short period due to their unstable form and.....No.....NOOOOOOO! GLARBLRAWRGLARBL
Only Failbaddon can comprehend how truly goddamn useless these things are. Generally by looking in a mirror. Khorne only knows why he hasn't been turned into one yet.
The most significant you-know-what in Warhammer is Scyla Anfingrimm. The only you-know-what who yet has the favour of a god.
 Why These Units Used to Suck
You-know-whats were just absolutely horrible before 6th Edition. Costing the same as three actual Chaos Marines, they would wander around the board aimlessly with a throw of the Scatter Dice, moved D6, had the Rage special rule and died to Bolter fire earning your opponent a free kill point. They fucking sucked, essentially. There is considerable evidence that they were originally supposed to have the Feel No Pain rule, ergo making them marginally useful, but sadly the 4th Edition Codex, through a series of misprints, lacked this.
 What These Units Are Like Today
Phil Kelly remembered that under that mass of writhing limbs and stupidity, there's a Chaos Space Marine, so brace yourselves for a shocker: they're not absolutely worthless anymore. New features include...
- A lower point cost (30, as opposed to the previous 40).
- Causing fear while being fearless themselves.
- No longer moving randomly or at a goddamn crawl. In fact, they move as fast as Metal Bawkzes and do not give a shit about terrain.
- Rage now meaning that they get +2 attacks on the charge, making it actually useful.
- A D3 roll every turn that determines what mutation they get for that turn. They can get a 4+ armor save, a roll of 2D6 for their number of attacks (you pick the higher one), or poisoned attacks (at 4+ to wound).
Their stats are otherwise unchanged, so holy shit, it's almost worth taking two or three of them and throwing them headlong at the enemy as a multi-wound distraction to soak up fire for you. Almost. Okay, that is not true. 30 points for a model with 3 T5 wounds that can move 12", has useful special powers, can be taken in squadrons and can become T6 with Nurgle is frankly excellent. Unless you run into Grey Knights or Dark Eldar, for whom the Spawn is just a mild annoyance.
 The other way they are useful (aka you need them now)
In the new Chaos Space Marines Codex there's a table, like the one in the Warriors of Chaos Army book, on which your HQ, when killing an enemy character (keep in mind: not IC, just C). It's a 2D6 table with some of classics: +1S, +1T, +1W...
But one of the results... forces you to swap your awesome super killy Chaos Champion with... err, the-one-which-shall-not-be-named! So now you have to buy a box of these sick horrors in order to play.
At least for Chaos Warriors, the time when your Marauder Chieftain turns into a Spawn and still does more damage than it would normally is truly priceless.
Cue rage and threats to eviscerate every GW employee that isn't Phil Kelly. But really, shut the hell up you fucking newfags; 'bout time you had to deal with this Chaos Spawn shit anyway...
Ohh Gods no waiiii.... RAWRAWRASDAFGSFDS (Automated Message From Cadia: We are out of Useable speakers and will be employing new ones at the Recruitment Center nearest you)
 Warhammer Fantasy
In Warhammer Fantasy, Chaos Spawn are mutations that only afflict those who worship Chaos (allowing the faithful to say their name without fear!). Exposure to the Warp for the faithful (or unaligned) instead causes mutations that corrupt the afflicted into more feral forms. Usually these mutations occur amongst common citizenry when the Winds of Magic blow too strongly from the miniature Eyes of Terror at the poles of the world, and patrols are required to cull them before they join the ranks of the forces of Chaos, although who it is that leads the purges differs from race to race. The Empire relies on the familiar Inquisition and Witch Hunters. Kislevites instead rely on their Dvoryanin and Boyars (nobles) to account for the status of the outlying populations. Bretonnian nobles consider all the peasants to be little better than this usually, and said peasants usually self-police due to their extreme devotion to the nobility (however, simply taking all babies born with mutations to leave in the forest is what caused the Beastmen to grow so prolific in the first place). Elves (High, Wood, and Dark) are near immune to mutation, as are Ogres to a lesser extent unless they serve willingly. Lizardmen and the Undead (Vampire and Egyptian) are both immune to the power of the Warp, with the latter actually able to reverse-corrupt things.
Chaos Spawn occur far more often amongst Warriors of Chaos than Chaos Space Marines due to their EXTREME fucking devotion to the Chaos Gods, to the point that every last Norseman seeks to become a Daemon Prince and the Chaos gods spread their blessings liberally. These spawn are usually used somewhere between biological weapons, and the same roles used by dogs depending on the degree of the mutations.
Chaos Spawn fear symbols of Sigmar and similar gods, and the power of nature as channeled by the Wood and High Elves can cleanse mutations from a subject. In particular, a book series depicting High Elves fighting Dark Elves and Warriors of Chaos describe how the Champion of Slaanesh prior to Sigvald the Magnificent is cleansed of all Slaanesh's blessings by simply touching the Everqueen. Slaanesh, very VERY pissed, began pouring his Daemonic energy directly into the Champion. When the Everqueen let go of his hand, that energy immediately turned him into a Chaos Spawn on par with a Greater Daemon.
Like in 40k, Chaos Spawn are terrible. They have low stats, slow movement speed with the inability to march or charge things (the latter would eliminate almost all thins in the game from being useable anyway). Their Marks of Chaos are random and cannot be counted on even working. Sadly, you'll probably need a a few to run fun things in Warriors since there's a chance of failure to become one for some of your options. In addition, some choices of Lords or Heroes come with some as pets (Cultist-chan should really be the poster child of Warriors rather than 40k cultists).