As the Chapter's High Chaplain he holds the title of Redeemer of the Lost. This loosely translates into that if you're overcome by the Black Rage to the point where you can no longer serve in the Death Company Astorath comes along and chops your head off with his massive axe. Most Blood Angels do not feel comfortable around him for this reason: it is very well possible that they are the next to receive a Viking crew cut from the Redeemer of the Lost. While some believe that Astorath causes the Blood Angels to succumb to the Black Rage this is far from the truth.
Astorath has a knack for sensing when the Black Rage is about to strike, and meets up with the brothers who are about to fall so he can lead them into glorious battle one last time. He makes no difference of Chapter in this, as Astorath's duties are beyond his own: he travels across the battlefields of all the Chapter's successors: so one month he can be fighting alongside the Flesh Tearers, and the next he is aiding the Lamenters. No matter the distance, Astaroth can feel the encroaching Black Rage in any of his battle brothers before even the brother himself starts feeling the effects.
This sometimes causes derp, like in the Shield of Baal books when it's mentioned that he once started detecting the Black Rage in a chapter that claimed to be of Ultramarine descent. The operative word here is 'claimed'. There's a ton of Chapters who don't know their Primarch, or their records have been lost, and they basically throw a dart at a dartboard, and proclaim themselves sons of whatever Primarch the dart hits. There's even records of the official Terran list of Astartes saying that a Chapter is descended from one Primarch, but the Marines themselves claim to be descended from another. Such was the case of the Griffon Lords, who claimed to be descended from Guilliman (Emperor knows why they chose him), but the official on Terra stepped in and said "Uh, you're actually from the White Scars. Jaghatai Khan IS the father!". Of course, this didn't stop them plugging their ears and saying "NUH UH, KHAN'S NOT OUR REAL DAD!" until the Terrans went away. It's entirely possible that there's a Chapter who think they're descended from Guilliman, and just are SO loyal that they sometimes skitz out, get a bit TOO loyal and start drinking the enemy's blood.
Of course, they probably wouldn't like Astorath rocking up, starting to behead them and then say "No no no, it's cool, it's cool. You're sons of Sanguinius." Turns out that said Chapter (the Carmine Blades) were pretty OK with the revelation, and in fact had been suffering from the Red Thirst for some time (which they believed to be some kind of curse). Of note is that they were originally named the Swords of Haldroth, and were renamed the Carmine Blades AFTER the discovery.
Astorath is essentially a chaplain on steroids, and in the humble opinion of this author he has one of the best looking models the BA have to offer, which is really saying something. For rules he comes with Angels of Death and Jump Pack Assault. Redeemer of the Lost allows all BA units within 6" to use his LD of 9, and makes DC units fearless. Has the MASTER OF SANCTITY keyword, and not only knows one more litany than the average chaplain, but can also recite an extra litany per battle round. BUT the good stuff is Mass of Doom. once per battle (which sucks when this fails) Roll a dice for each friendly Blood Angels infantry unit within 6", on a 1 they take a mortal wound, 2-5 add 1 to all hit rolls, 6 same as before but they get 4+ invuln save. Stick him in a unit of jump-pack Death Company, charge them at something, and everybody dies. Everybody.
He is musingly named as a variant spelling of Ashtoreth, which itself is the demonic name for Astarte who was a deity of war in many of the ancient Semitic religions of the Middle East. Furthermore; Astaroth (and many other derivations) are often associated with various deities named "Baal" after whom'st the Blood Angels home planet is named.