So here's a chapter I imagined following my adherance to the Dark Angels army

Sorry if there are errors in my words (english is my second language)
Summary
The Dead Angels are a successor chapter of the Dark Angels, and as such are part of the Unforgiven Space Marines chapters. The thing that make them unique compared to their fellow Unforgivens is that all its members are aware of the horrible truth concerning the Fall of Caliban and that it is their sacred duty to make sure no one outside the members of the Inner Circle become aware of even a glimpse of information about it. They are in a way the secret police of the Unforgiven, always acting in the shadows of their brothers and ready to strike anyone that can become a threat. However, their duty do cover more than the successors of the Dark Angels, and they often come in conflict with anyone that learn to much. In this regard, they sometime even have to battle and exterminate members of the xenos race, and even members of the Imperium if it is needed. They take great care to leave no trace behind them that could reveal their existence or the truth about the fall of Caliban. Because of their duty, they do not actively participate in the hunt for the Fallens, but if the occasion present itself, they can act but only if it do not endanger their first mission.
The Dead Angels are true angents of Death, leaving nothing alive after their passage, and killing without remorses. To them, the duty come first, and the morality of an action is judged by its contribution to it. The only member outside of their chapter to know their existence is Azrael, Supreme Master of the Unforgiven Chapters. It is him that direct them where to go and what to do, andf they only report to him outside of their own chain of command. However, their loyalty lay to the position of Supreme Master of the Dark Angels, and not to Azrael himself. Should ever he become a traitor of endanger the sacred duty of both the Dead Angels and the other Unforgivens, they have the sacred obligation to neutralize him and make him disappear by any means necessary.
The Dead Angels follow this combat doctrine : By any means necessary, do what must be done in regard of the Duty. In combat, Dead Angels Space Marines never retreat and go so far as endangering themselves and their allies if it serves the cause. Thsi make them dangrous allies because if the sacrifice of their own allies can serve them, they will make it happen.
Dead Angels armors are jet black and white, and most of them wear ceremonial robes in combat. They are an extremely religious organization, passing much of their time either to battle, train or participing in religious rites. Even their combat strategy have something of religious to it, their warriors thinking of the act of battle as a prayer to the Emperor and the Lion, and even shouyting prayers will slaughtering or be slaugthered.
The Dead Angels do not follow the standards of the Codex Astartes in reason of their Doctrine of ''By any means Necessary'', and as such could be considered heretics if ever discovered. This is also a reason why they make sure to leave no witness of their acts behind them.
Key Figures
Chapter Lord Uriel : Uriel is the master of the Dead Angels, a position he rose to by being selected by Azrael from amongts the other Dead Angels. Uriel is famous for always leading the chapter in battle even in its most desperate moments, and there is nothing more appealling to him than do deliver death to the enemies of the Unforgiven or die trying. On the battlefield, Uriel bears his consecrated terminator armor and his feared pair of lightning claws, bringing dead and woe to his enemy by himself. As Chapter Lord of the Dead Angels, he is also the master of the DeathWave, the Deathwing counterpart of the Dead Angels chapter.
High Chaplain Cyrus : The religious leader of the Dead Angels, Cyrus make sure that no heresy infect the mind of any Dead Angels Space Marines, and make sure that each of them is totally devoted body and soul to the Duty. This can make him threat new recruits in ways that could seem to be torture for any outsider, but Cyrus goal is really to make sure that no Dead Angel will ever betray his duties. This can take the form of physical or psychological torture where the recruit is tested by telling him the only way for him to survive his by revealing the chapter secrets to a figure that pass for an enemy. Cyrus take his task really seriously, and no recruits can tell whenever they are tested or not. Sometime, a recruit that think he has been captured by enemies who torture him for information discover that in fact this was just all part of a big scenario to test his loyalty. Only those who have achieved to pass these test are welcomed as true Dead Angels and are then admitted to go on real battlefields. Those who fails are then delivered the Emperor's Justice by a olt in the head.
Grand Librarian Archemedius : The supreme Psyker of the Dead Angels chapter, Archemedus is the keeper of forbidden lore concerning the chaotic enemies of the Imperium. It is also him that decide who can and can't become a librarian in the chapter. Those he accepts become mighty psykers, those he refuse are eliminated to make sure they do not use their power for evil causes. He and Cyrus are a little at odd, for the second see the first as an heretic tolerated by the Imperium, while the first see the second as a conservative simple-mindeed being. However, both work with each other for the sake of the Duty. Whenever Cyrus needs to have a second opinion on a recruit he just tested, he goes for the help of Archemedius. When Archemedius needs to be sure if a recently obtained knowledge must be destroyed or preserved, he goes for the help of Cyrus. However, both of them prefer it when they don't have to need the help of the other.
Relations
Space Marines : The Dead Angels consider the Dark Angels and all other Unforgiven Chapters as the ''true'' space marines because of the rumour that the Dark Angels were the first Legion to be made. As such, they consider all non-Unforgiven chapter as being made-up of flawed imitations of what a Space Marines should be. In this regard, they see other space marines as nothing more than tools that can be used and destroyed if needed.
Imperial Guards : The Dead Angels look at them as if they were nothing but a meat shield.
Grey Knights and the Inquisition : The Dead Angels avoid all contact if possible with these organizations. If their existence could even be sensed by them, almost nothing could be done to preserve the secret of the Duty. If ever Dead Angels come in contact with any memeber of the Grey Knights or of the Inquisition, the order is to silence them immediatly, for leaving them alive is to great a risk (in reason of their power and ressource)
Chaos Space Marines : The ultimate enemies with which no pact is tolerated. Even thought the Dead Angels have a ''by any means necessary'' doctrine, collaboration with the heretics is absolutely penalized with death sentance by anyone that dare cross the rule, be it Dead Angel or simple civilian. Dead Angels see Chaos Space Marines as the perfect example of what might happen if they ever fail in their duty of policing their brethren.
Eldars and Tau : Dead Angels are ready to colloborate with these xenos scum, but when the work is done, they will make sure their secret of their existence is safe by any means necessary.
Tyranids, Orks and Necrons : No collaboration is possible with these aliens, and by such they must be exterminated at sight. For the religious Dead Angels, these abominations aren't better than daemons of the warp. However, the Dead Angels are partially intrigued with the Necrons technology that could seal of permenantly the immaterium from the materium. However, their religious belief stop them from ever truly using it.
Daemons : The source of the Burden. To destroy the Fallen is but the first step in making sure such a thing as the Fall of Caliban ever happens again. The final is the permanent elimination of the Chaos Gods.
To resume : the Dead Angels are ready to work with anyone that isn't to much alien, but once the work is over, they will backstab them to make sure no one even know they were there. For they rest of the Galaxy, they must be nothing more than nothing if they hope to accomplish their goal.
Combat Doctrine
Dead Angels are relatively few compared to other Space Marines chapter, the reason being that they don't often have to go to combat and that they specialize in fast special operations. However, they are highly trained in the art of war, and most of them are challenged by a deadly training. Most Dead Angels that are alive have survived trials that make war look like vacations in a hot spa summer, and each of them is always at risk of dying during training, for the training is never completed. Even the most battle-hardened veteran must continue to train when not at battle or participating to the religious rites.
In battle, the standard tactic of the Dead Angels are quick and lethal strike. First go the bikers with homers and soon after ward the dreadnought and devastators in drop pods come in, followed by the terrifying terminators of the Deathwave.
Of course, the Dead Angels have the doctrine ''by any means necessary'' which make them never retreat, always using all the options they can find. If the only way for them to win is to litterally go to death, then they will. This berzerker-like attitude as often been costly, but since they are a black ops-like army, they have more than plenty of time to refill their ranks, and much of the time only a small battleforce is necessary to do the job.
In battle, the Dead Angels favor close combat, seeing something religious in the attack of gutting the enemy with blades and claws. However, they understand they will not always be facing simple soldiers, but also huge warmachine, which is why they are often equipped with melta weapons.
So here's for now, I will continue later !