It is a really simple back compatible insert.
They are a faction of the inquisition (ordo xenos)
As such use the allies matrix found in the witch hunters/demon hunters codex(it is the same thing) or run them as their own force. remember that 3rd-7th or cross compatible with the same basic core mechanics, just adhere to the rules in the edition you are using for the base.
Our group uses 5th ed as core but i also own a small force of deathwatch (and solmon Lok i bought over a decade ago from
FW just because i loved the model) with the 7th ed codex that i bring as allies from time to time.
I did this battle with dark angels (3rd) and allied deathwatch (7th) when fighting an eldar force.