i guess this spoils the new deathwatch quickread "Flesh of the angel" as that is where this info came.

* It states they came to the planet and invaded the areas to make their breeding pool.
* The Sslyth seem to have deep underground lairs where there are thousands of eggs.
* They seem savage in nature, reminding me a bit of "predator".
* one seemed to be decorated in human bones/eyeballs ect and was wearing leather clothing.
* others seemed more warrior-like and carried swords.
* one is noted to be carrying a large bolter, and others wore stuff made from astra militarum soldiers armour/gear.
* later it's determined the ssylth have had contact with the emperors children.
* the sslyth are described as alien mercenaries who had plagued an entire sector for centuries as pirates.

personally id love to see them expanded a bit as allies for mostly DE and Chaos.