Chaos Spawn
The Chaos Spa-, ahem, the unit in question are considered by many to be the worst unit in the Chaos Space Marines codex and, quite possibly, the entire game of 40k. They are so bad, in fact, nobody takes them, even 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 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 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 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.
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. There is considerable evidence that they were originally supposed to have the Feel No Pain rule, ergo making them marginally useful, but sadly the Codex, through a series of misprints, lacked this.
The Only Way They're Useful
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 Sp-, uh, you-know-what - because it's fucking hilarious. Should you successfully employ it against Kaldor Draigo, the Chaos Gods will laugh hard enough for it to be audible from Catachan.
Note that due to the fact that the spell can be used in close-combat and resolves on toughness, it is best-employed against lower-toughness high-value targets, such as Guardsman and Eldar commanders. It is of acceptable use against Marine equivalents, since it ignores armor and invulnerable saves. Employing it against a low-toughness unit such as Grots is a quick-and-dirty way to completely butt-fuck a Tarpit given some lucky rolls. It's a very finicky spell, however.
Which is the one way that Spawn can be usef.... Ohh Gods no waiiii.... RAWRAWRASDAFGSFDH (Automated Message From Cadia: We are out of Useable speakers and will be employing new ones at the Recruitment Center nearest you)