Officio Assassinorum
When things get really fucked up, this guys are who clean the shit. The officio assassinorum was founded at the very end of the Horus heresy as a way for hold any possible heretic and overlord wannabe in his place until the Age of Apostasy (a.k.a Age of trolling and spacedrama) make the Holy Terra senate be the only ones who can do the votation necessary for order any assassination because they are too badass for let them be around without control.
The Grand Master of Assassins is the head of this organization and he matches in political power people like the Fabricator-general of Mars and even the Ecclesiarch of the Adeptus Ministorum to the point he can say stfu to them without any repercussion. Their people are recruited mostly from the orphaned sons and daughters of officials in the Imperial Guard but also pick people from dead worlds where babies eat devourer swarms instead of suck milk from their mother's breasts, and trained in Holy Terra's school of assassins until they do their major on one of the specialist temples.
Then is when they leave bad ass level to go Chuck Norris level to the point of make a Space Marine have nightmares, because they even dare to use banned technnology from the Dark Age of William Gybson and xeno artifacts for their missions, like synskin (what was developed by leather and rubber fetishist in the pre-Imperial era) or the C'tan phasic sword.
Consists of:
temples.
Death Cult assassins wish they were in it.