I suppose you could say the Mythic Dawn cult were Mehrunes Dagon's assassin.
I seem to remember a Khajiit society that was half guerilla fighter and half assassin (each member that is), but they were crazy.
I also suppose that the Thalmor and Akaviri must have had independent assassins too.
People believe what they either want to be true or what they are afraid is true! Wizard's first rule
Passion rules reason Wizard's third rule
Mind what people Do, not what they say, for actions betray a lie. Wizard's fifth rule
Willfully turning aside from the truth is treason to one's self Wizard's tenth rule
i think those black daggers and wormcult dudes are awesome. but as far as lore, there was basicly the offset of thieves guild people who just stole items from you and tried to kill you if you saw them steal stuff.
there was also the sixth house members in morrowind.
and also there was definite ashlanders who would kill you as a treat just because you come near thier grazeland tents and huts.