The Betrayer, also known as Commander Valen, is a former high-ranking member of the Night Sentinels, and is an important character in the backstories of Doom (2016) and Doom Eternal. Through the deceptions and machinations of the Hell priests, he became a central figure in the fall of Argent D'Nur.
As the elite warriors of the Night Sentinels began a foray into Hell to destroy the Nekravol and cut off the Argent energy which was corrupting their civilization in service of the Maykrs, only to be scattered and trapped in Hell by the Hell priests, another plot unfolded. Commander Valen had earlier lost his son in the struggles against Hell. The Hell priest Deag Grav, in concert with the demons, began to torment Valen with visions of his son suffering a tortured existence.
Deag Grav made an offer to tempt Valen. In exchange for the keys to the Elemental Sepulchre, where the Sentinels' ancient gods—the Elemental Wraiths—slumbered, Valen's son would be resurrected. Said to have buckled in a "moment of weakness," Valen consented and led the priests and their forces to the Wraiths.
Seizing the opportunity to strike at the source of the faithful Sentinels' power, Deag Grav used the power of the demonic Crucible to slay the Wraiths. Their souls became the basis of a new source of Argent energy called the Well, which would serve the demons of Hell and bring them into a new age they would call the Ascension. Argent D'Nur was immediately overwhelmed by a massive demonic attack. Bereft of their Sentinel energy, the remaining faithful warriors under King Novik could not resist. The corrupted Argenta of the Ascended Empire, under the leadership of the Deag priests and the Maykrs, would come to rule the remaining portions of their world exclusively.
As part of this final invasion, a new Titan came to Argent D'Nur - the Icon of Sin. It had been created by combining the ancient Titan with the heart of Valen's son, whose soul had been bound within his heart, and the heart then bound to the Titan. The Icon of Sin, later described as a world-devouring super-predator, would consume parts of Argent D'Nur wholesale and draw them into the dark realm to be forever corrupted. This was Hell's twisted manner of honoring its deal with Valen.
Valen henceforth became known as the Betrayer. As self-inflicted punishment for his sacrilege, he banished himself forever to the Hellscape. His fortified dwelling can be found not far from the Nekravol his comrades died fighting to destroy. In the course of Doom Eternal, the Betrayer helps the Doom Slayer by powering up a component for the Celestial Locator, a device which can find the Hell priests. In addition, in hopes of laying both his son's soul and his own to rest, he gifts the Slayer a special dagger designed to destroy the heart of the Icon of Sin. The Slayer later uses this weapon successfully at Urdak, preventing the Khan Maykr from taking control of the Icon.
The Betrayer has dark skin, wears his white or gray hair in a flat and narrow mohawk, is heavily scarred from ages of combat, and is blind in his right eye, which is whited out. He wears a suit of armor similar to the Praetor suit, including a shoulder-mounted equipment launcher. His primary weapon is a massive Argent hammer.
As the Betrayer has forge-like equipment in his encampment and many weapons nearby, and is apparently one of the Slayer's primary sources for Sentinel technology parts and repairs, it is possible that the Betrayer is also the one known as the "wretch" by the demons, who is said to have forged the Praetor suit in the fires of Hell. This remains unconfirmed, but there are currently few other candidates who could qualify, given the requirements of location in Hell and timing after the fall of Argent D'Nur.