My understanding,
DOOM 1, 2, 64: William BJ Blazkowicz II, descendant of the protag in Wolfenstein 3D
DOOM 3: John Kane
DOOM Comics: Flynn Taggart
DOOM 4: Doom, Doom Slayer, Unchained Predator, Hell Walker, Scourge of Hell
Literally just known as "doom" or "doom slayer" by the inhabitants of hell, originally a denizen of alternate dimension Argent D'nur, he was the leader of the 'night sentinels', an extremely powerful band of warriors that defended Argent D'nur's incredibly large source of power: the wraiths. The green ghosts you see at certain parts in hell are the ghosts of the night sentinels.
After having lost his son in battle the Doom Slayer betrayed the people of Argent D'nur by leading demons from hell to the wraiths. The demons merged Argent D'nur with hell and gave the betrayer his son back, but in a hideous demon form known referred to as "the icon of sin" (sound familiar? It's the final boss of DOOM II). This made him, quite frankly, pissed as forget, and the doom slayer became so enraged that the maniac spent the next few
millions of years doing nothing but tearing apart demons because he hated them so much. He pushed the demons further into the depths of hell because they were so loving terrified of him.
A mysterious angelic like figure saw his rage and gave him an immensely powerful suit of armor, the praetor suit, to wage his war against hell (in the beginning of the game you hear someone say "They are rage, brutal, without mercy. But you. You will be worse. Rip and tear, until it is done", it is theorized that this is that being).
The slayer even defeated something referred to as 'the champion of hell', you see the massive skeleton of it in the Titan's Realm mission. Eventually some demonic priests lured him into a booby trapped temple, and collapsed the entire structure on him, and then trapped him into a cursed coffin that put him into hyper sleep, the Kadingir Tomb. During the Kadingir expedition, UAC explorers led by Samuel Hayden found the tomb and brought it back to Mars as a safeguard in event of demonic invasion. When Olivia Pierce opened a portal to hell in the UAC facility on Mars, the Doom Slayer awoke, and then the events of DOOM 4 take place.
He is pain, he is hatred, he is anger and rage incarnate, he is DOOM, the bane of hell and it's slaves
"In the first age, in the first battle, when the shadows first lengthened, one stood. Burned by the embers of Armageddon, his soul blistered by the fires of Hell and tainted beyond ascension, he chose the path of perpetual torment. In his ravenous hatred he found no peace; and with boiling blood he scoured the Umbral Plains seeking vengeance against the dark lords who had wronged him. He wore the crown of the Night Sentinels, and those that tasted the bite of his sword named him... the Doom Slayer.
Tempered by the fires of Hell, his iron will remained steadfast through the passage that preys upon the weak. For he alone was the Hell Walker, the Unchained Predator, who sought retribution in all quarters, dark and light, fire and ice, in the beginning and the end, and he hunted the slaves of Doom with barbarous cruelty; for he passed through the divide as none but demon had before.
And in his conquest against the blackened souls of the doomed, his prowess was shown. In his crusade, the seraphim bestowed upon him terrible power and speed, and with his might he crushed the obsidian pillars of the Blood Temples. He set forth without pity upon the beasts of the nine circles. Unbreakable, incorruptible, unyielding, the Doom Slayer sought to end the dominion of the dark realm.
The age of his reckoning was uncounted. The scribes carved his name deep in the tablets of Hell across eons, and each battle etched terror in the hearts of the demons. They knew he would come, as he always had, as he always will, to feast on the blood of the wicked. For he alone could draw strength from his fallen foes, and ever his power grew, swift and unrelenting.
None could stand before the horde but the Doom Slayer. Despair spread before him like a plague, striking fear into the shadow-dwellers, driving them to deeper and darker pits. But from the depths of the abyss rose The Great One, a champion mightier than all who had come before. The Titan, of immeasurable power and ferocity. He strode upon the plain and faced the Doom Slayer, and a mighty battle was fought on the desolate plains. The Titan fought with the fury of the countless that had fallen at the Doom Slayer's hand, but there fell the Titan, and in his defeat the shadow horde were routed.
And in his terrible rancor between worlds and through time, the Hell Walker found the wretch who shall not be named, but in his heresy was loyal to his evil cause. The wretch adorned the Doom Slayer in a mighty armor, wrought in the forges of Hell, impenetrable and unyielding. With sword and shield of adamantine strength, the Doom Slayer set to banishing all that were left unbroken by his savagery to the void.
Yet as the mighty Titan fell and dread engulfed the armies of Doom, the demon priests of the Blood Temples laid a trap to capture this scourge of Hell. Insatiable, even by the vanquishing of the Great One, the Hell Walker sought prey in the tombs of the Blood Keep. And blinded by his fervor, the lure drew him in. The priests brought down the temple upon the Doom Slayer, and in his defeat entombed him in the cursed sarcophagus. The mark of the Doom Slayer was burned upon his crypt, a warning to all of Hell that the terror within must never be freed. There he lies still, and ever more, in silent suffering."
