Dark Angels Grand Master
The Dark Angel's rough equivalent of a Brother-Captain and/or Chapter Master, though the role appears to have more to do with their position within the Inner Circle rather than their military rank and as such they are privy to more of the secrets regarding the Fallen Angels.
Contrary to most Chapters which are led by Chapter Masters, the Dark Angels and their Successors maintain close ties, bound by an Inner Circle of Grand Masters sworn to hunt down the
Fallen FOUL TRAITORS AND HERETICS FROM 9 TRAITOR LEGIONS THAT HAVE NO CONNECTIONS TO THE FIRST LEGION WHATSOEVER!
Unforgiven Grand Masters are often given leadership over specific duties and organizations, The Dark Angels themselves maintain six Grand Masters: Belial of the Deathwing, Sammael of the Ravenwing, Ezekiel of the Librarius, Sapphon of Chaplains, Razaek of the Apothecarion, and Nephalor of the Fleet.
Other Unforgiven successor chapters also use the rank of "GRAND Master" to refer to the commanders of their 1st and 2nd companies which mirror the Deathwing and Ravenwing in function and mission, though it is uncertain how they award the position to other individuals within their own hierarchies, though it would be a safe bet to assume they follow the Dark Angels Chapter closely.
It's important to note that the normal Company Captains are simply referred to as "Master" and there is no requirement that they even be members of the Inner Circle at all. There are instances in the novels where Company Masters have been outranked by Deathwing Sergeants and Interrogator Chaplains when matters involving the Fallen have arisen, such is the way that the Inner Circle operates. This is also particularly true with the addition of Primaris Marines to the Chapter while the Unforgiven attempt to assess their loyalties, and only one example seems to have made it into the Inner Circle so far (to be fair, he was an established member of the Chapter to underwent the Rubicon Primaris rather than a fresh face).
The head of the Techmarines is also only a "Master", not least because of the dual loyalties of the Techmarines to the Priesthood of Mars which specifically disqualifies him from the role, but because he is physically hooked up into The Rock and cannot participate in the Hunt to any appreciable degree and the less know the better.
They should also not be confused with the similarly named Grey Knights counterpart also known as Grand Master, though Azrael himself continuously (and intentionally) confused the rank of Kaldor Draigo to make him out as his inferior.
Supreme Grand Master
Overall command of the Dark Angels Chapter and de-facto command of the entire Unforgiven is given to the Supreme Grand Master, always a member of the Dark Angels Inner Circle who is selected from among the Chapter's Grand Masters. Being selected is only a part of the ascension process, as with all things relating to the Dark Angels, the candidate must undergo trials to assess his suitability for the role, otherwise they are taken away and never seen again.
The Chapter Masters of Dark Angels successors are still only referred to as Grand Master; the title of Supreme Grand Master exists solely within the Dark Angels as the head of the entire Unforgiven. While this might sound like it can cause issues of uncertain authority when Captains hold the same title as their Chapter Master; the rank also tends to come with additional honorific titles such as "Master of Souls", "Finder of Secrets", or "Holder of the Keys" etc etc, which indicate their role within the Inner Circle. But chains of command within Space Marine Chapters are short enough that they would be able to identify their superiors easily enough even without standardized designations of rank.
Anyways, Azrael is the present Supreme Grand Master of the Dark Angels
Legion Chapter and he carries the honorific title of Keeper of the Truth as he is that trusted with the Chapter's most closely guarded secrets.