Chaos Spawn
The Chaos Spa-, ahem, the unit in question are considered my many to be the worst unit in the Chaos Space Marines codex and, possibly, the entire game of 40k. They are so bad, in fact, nobody takes them for flavor or joke games and 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 refer to as you-know-whats. That or they-who-shall-not-be-named.
In fluff, aspiring champions of chaos normally devolve into said units if they use chaos for their own gain, their patron God thinks they're not worthy 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 Failaddon can comprehend how truly goddamn useless these things are. Generally by looking in a mirror.
Game play
You-know-whats are horrible in-game. Coming in at a whopping forty points (That's almost three normal CSMs) they wander around the board aimlessly with a throw of the Scatter Dice, move D6, have the Rage special rule and die to Bolter fire earning your opponent a free kill point. They fucking suck, essentially.
Still though, if you play a Chaos Sorcerer and you're cast-happy with the "Gift of Chaos" skill, you might want to keep a few of these models in the event you managed to turn one of your opponent's guys into a said creature who will remain nameless. And Chaos grants favor upon you if you managed to turn a high-point model like Marneus Calgar or Failbaddon the Despoiler into a Chaos Spawn.....ohh Gods no waiiii.........RAWRAWRASDAFGSFDH