I am unfamiliar on them. What are their weapons and capability like? Equal to imperium in places? Would the Deathwatch know much about them?
I'm sure the Deathwatch knows about them. They're just not known in the Jericho Reach. But they are known in Koronus. Which opens the scary possibility of them breaching the security of the Warp-Gate to get here.
They are frankly horrifying with stats. They probably can 1-shot RT characters given the chance. Their solos really bloodied the fully-equipped marines I sent against them. The marine who was sans armor had to play it smart and stealthy to reunite with his team or he would have been killed.