Agent23
Ни шагу назад!
They are a highly trained, highly organized unit of battle brothers that come from various chapters to act as the Chamber Militant of the Ordo Xenos.Depends how are they being organized? They'll easily slaughter basic Yoma for years but might last minutes to seconds against any Abyssal.
So they are the best of the best.
Probably not on par with Grey Knights one on one but close IMHO.