Imagine the kind of Marine that has to lead the Lamenters. The kind of noblebright yet hopelessly doomed son of a bitch that is considered the best among one of the most unluckiest, unwaveringly fucked-over chapters in the history of the Imperium of Man. That has lived for centuries only to see his brothers mistreated, killed, and exiled. Imagine what it would be like to be that man.
What you are imagining is what it is to be like Malakim Phoros every single fucking day he's alive.
Malakim Phoros is the lucky bastard who gets to be Chapter Master of the Lamenters. He holds the title of "Lord of Ruin", for what hangs over this man is the unshakeable aura of grief for his many, many, many brothers lost, and the deep, bitter hatred of everything that led his chapter down the road they're on, which only seems to give his fellow Lamenters an even more vicious determination to absolutely fuck up their opponent's day (and their own, if we're being honest with ourselves). He also almost certainly received this title for being the man to have his chapter agree to follow the Astral Claws in their little secession project from the Imperium of Man. And we all know how that went.
Phoros as a person and leader is a complicated figure. He is shown nothing but absolute awe and reverence by his battle brothers, for he is in many ways the model Lamenter: A picture of serene Sanguinian beauty, his personality perfectly reflecting his chapter's concern for their charges of the Imperium, and his progenitor chapter's beautiful artificing in his exceptionally crafted gold armor. But deep down inside, Phoros has long since held the bitterest of frustrations with the Imperium's leadership for both perceived rank incompetence and their castigation of his chapter, the emptiness of being a Chapter Master for a Chapter nobody save their fellow brothers in the Blood Angels and the Ultramarines truly respect, and has been at war with himself in a very literal fashion; as he is nearly constantly teetering on the edge of falling to the Black Rage. He's almost fallen to it a number of times in the heat of battle, but has dragged himself back from the brink each time. Phoros knows however, that this is ultimately a losing battle, and that each time he's done this, a little more of his sanity erodes each time, and that one day he will at long last succumb to the Flaw, and be lost to the Lamenters forever.
Phoros has long since been considered KIA by the Minotaurs when they crushed the Battle Barge Daughter of Tempests like a soda can during the Badab War, but rumor persists that Phoros is alive, having been recovered out while he was floating out in space, powerless to help his chapter survive the onslaught of the High Lord's lap dogs, and continues to assist his Chapter up to this very day. Given his already frosty relationship with Imperial authorities, especially after what his Chapter got handed in the aftermath of Badab, it's unlikely he'll ever answer a hail from them personally ever again; likely to give his Captains a chance to see the realities of leadership in case he finally snaps for good.
On the Tabletop
Phoros has been given rules periodically by Forge World in the Imperial Armour books all the way back in 5th edition, and has been reprinted every so often in the same books.
In 9e, he's been dumped into Legends, essentially acting as a suped-up Can opener at 145 pts with an extra attack and wound, with the usual Chapter Master goodies like an Iron Halo, Some captain abilities, with a glaive encarmine and an inferno pistol, both with a -4 to AP. His Lord of Ruin ability allows him to re-roll Wound rolls of 1, and Rage Unto Death allows him to charge on turns in which he Fell back.
In typical Lamenters fashion, he had a bunch of things that set him apart from the rest of the pack, like a crazy Stealth-Armour rule that gave him a +1 to his armor saves against ranged attacks while in cover and an aura that granted Lord of Ruin to his fellow battle-brothers within 6", but he lost those in a couple of Erratas, as well as receiving a 30 point price hike. He's still a pretty solid choice of Chapter Master, especially for those poor boys in yellow, but you still have to dump him in a transport to properly get the most out of him.