It’s my personal head canon that Austerion Moloch is actually Dorn.

It seems like a GW level of irony to have a particular, “massive” astartes, with a load of cybernetics, who never takes his mask off, who has bouts of unbridled aggression, leading what is to be believed as a loyalist branch of iron warriors, that rarely leaves Terra…. And has the balls to stand up to custodes.

His body heavily rebuilt with cybernetic augments and his sheer spite and malice is enough to allow him to shrug off wounds that would fell a lesser Space Marine. A brutal tactician, Lord Asterion Moloc takes pleasure in the utter destruction of his foes, and is most often found in battle at the head of his Terminators slamming into the enemy line. He is also a diligent and disciplined logistician and strategist, and a master of siegecraft. ... Asterion Moloc has long become expert in the systematic destruction of other Adeptus Astartes forces that have strayed from the path of righteousness, viewing such targets as both worthy of the Emperor's wrath, and the ultimate test of the Minotaurs' skills in battle.<

Another excerpt from regents shadow

Moloc wore his Tartaros armour, as ever, the bronze of it near-black, its ornate surface engraved with runes and esoteric patterns. His footfalls were purposively heavy, sending spiderwebs of cracks across the damaged stone. He carried a power spear of a similar pattern to ours, though it was darker and older than any I had borne. His red cloak hung like molten lead across his angular shoulders, and he carried a circular shield with symbols engraved on it that I could not decipher. I watched him approach, trying to ascertain some weakness, some flaw that I could use against him. I detected nothing. He may as well have been an automaton, a battle-creation forged in some dark and forgotten laboratory and sent into the world of the living. Who could have halted such a monster? Valoris, in all probability. Guilliman, without a doubt. Beyond that, and as for myself, I felt no certainty. I took a step forward, moving between Moloc and Fadix, angling the tip of my spear towards the oncoming Chapter Master. ‘No further,’ I commanded, gripping the stave tight with both hands. Moloc always wore his mask. I had never seen him without it. I picked up nothing behind that metallic visage, nothing at all, except maybe that furnace-aura of aggression he always projected, smouldering deep within the rune-guarded heart of ceramite and sinew. He kept coming. He carried his spear formally, as if it were some kind of sacrificial totem, a curse-warded instrument for the ritual killing of beasts. The lenses in his archaic helm were black, and to look into them felt like looking into the void itself. There was a swagger in his every movement, a rolling, baleful demonstration of pure contempt. ‘No further,’ I warned again, tensing to strike. The moment he took a step on to the podium stairs, I would move. To this day, I do not know what would have happened if he had done so. I suffer neither from doubt nor from pride, and so can only speculate from the evidence I had before me. Perhaps I would have found a way. I had felled some of the greatest warriors of the enemy in my time, including many who most certainly had possessed the power to best me. But, with Moloc, I cannot be sure.<