hey. im having some problems keeping my heretics loyal to there boss. they where put in charge of sacgrave by a Black Legion commander and where placed in the lost and damned once an impieral crusade entered the screaming vortex. however they are lead by a human in the lost and damed. even though the space space marine hasnt forced them to do anything they still are trying to oust him.

the main reason i need him is in order to continual give them missions.

I have found that in many cases unless the PC's are working under a individual that is overwhelmingly more powerful than them, giveing them a boss like there members of the deathwatch or guardsman or imperial acolytes is generally tough. instead of trying to have someone boss them around I find it best to use the characters motivations lead the campagin, find out what there characters want to do and build the adventures around that, even useing the pc's as a source of information. ("in your studies of the warp you belive you have located the first piece of the demon relic now you just need to find a way to get at it")

like with a rogue trader crew its usually best to have your adventures set up to illistrate that the characters have the freedom to do what they wish otherwise they might as well return to the arms of the corps emperor.

I'd remind you this is a CHAOS game. Your own Black Legion taskmaster is by definition a member of a group called the TRAITOR Legions. If the players want to oust the boss, let them have their title shot. If they win now they have to face the difficulty of generating their own missions, drive and motivations. I run a game with two players. A future Slaanesh Champion and a future Tzeentchian Sorcerer (as soon as they hit 10 corruption). to learn the system I set them up where our 3 (i have a raptor who helps out with exposition and hints from time to time) Astartes worked for a warlord named Ascalon. he had droves of human slaves but only a handful or other Space Marines. I've made the concept of challenging him daunting by putting him in Terminator plate but it could be done. And now that they've been sent on a task on their own recognizance they have the chance to really take their destiny in hand. I've told them where we go from here is on them. They could continue to serve Ascalon. they could try and join up with a different warband we encounter. they could try to acquire their own warband. it takes a bit of both some planning and some off the cuffing but it can be done and I think its a bit more rewarding for me as a storyteller and my players if they're in a world that they shape and responds to them and even has a life all its own then your stereotypical go here, kill all the monsters, steal all the loot, high five, rinse and repeat per game session rpg.

Any good group doesn't need orders, they just need heavy hints of an opportunity, or offers of work from powerful groups etc.

Let them stage a coup or wahtever - give them control and let them look for work.

If you ned a boss, or just need to ensure they don't kick off on the next guy to offer them work, then make the boss powerful - have it so this is a job for someone who has bigger fish to fry, but would be well disposed to a group that sorts out this minor annoyance that they have.
I.e. "I'm busy levelling a planet right now, am half the sector away, and just don't have the time or people in the area, to find a missing transport, but if you could find it and bring it's cargo to me intact (the cargo taht is - the crew are expendable if they prove a problem) you will gain my favour and be well rewarded."

Well, you can always remove him by someone who is more cruel and tyranic... But my suggestion: if they really want to remove their boss, let them get a fair chance doing so... if they DO mannage to remove him from play they might move up the ladder and take his place, which means that they will have to prove themselves and get more resposibility.. and the higher-ups might actually approve, they are stronger = they should be in charge... Higher rank means more power, more responsibility and bigger dangers. Perhaps someone somewhere ivested a lot in the guy they remove and is secretly working against them.. And their comanders are bound to be watching their back even more, But if they are really devious and the dark gods favor them, they might find themselves in charge, even if the cost of their that is that they find their new organisation greatly weakend by their power struggle....

Ofc if they try to oust the guy and fail... they better start running... FAST

Don’t forget every group has a hierarchy and just because you cut the head off doesn’t mean that the sub-leaders will now follow whoever takes its place. What would most likely happen is that each of the sub-leaders will take their groups and either go off and do their own thing or try and take control of the other sub-groups.

And what happens when their boss's boss hears of his death?

Depends. Either the boss's boss takes note and is impressed or they executed for their defiance. He might not even care, one less person who could then betray him.

If a player wants to seize control or disobey orders then let them...but if they do it they either succeed and enter the unknown with even more potential risks, having no true guidance and opportunities, becoming minor insignificant heretics or they die trying and have to generate a new heretic.

Essentially while under the services of this master, it is at least a possible path to greatness and power. By removing them, they cut off that route to power and perhaps the number of contacts and options that staying on said route could bring. They must find ways to make their own destiny, suffer not having someone to perhaps watch their back on rare occasions or feed them information.

A good example seems to be in Breaking Bad. They take out several people over them in the course of the series, but either end up stepping on someone else's territory, pissing off the cartel and even when those obstacles get "solved", new ones keep happening, especially on the logistics side of things.

Point is: Just because you offed the boss, doesn't mean all the issues are over