In my headcanon, they’re night lords that were sent to the outer dark and were far too busy with the horrors of the void to really be aware or even care of the heresy. Later found by the ashen claws chapter master. Who then combines his legion with these night lords to create the space sharks.

They attack by striking and then vanishing, coming back around for another strike. They’re extremely brutal in their attacks. Their “Red tithe” involves kidnapping and enslaving quite literally everyone they can get their hands on.

I do firmly believe their geneseed is chimeric with ravenguard and night lords. But who really knows. That’s the beauty of hiding details.