Betrayer

"Saving your people will not bring you peace, only make the burden you carry worse."

- The Betrayer to the Doom Slayer

The Betrayer, real name Valen, is the former commander of the Night Sentinels that committed treason against the realm of Argent D'Nur to bring back his dead son. He is first mentioned in Doom (2016) and makes a full appearance in Doom Eternal.

History
Valen was one of the Night Sentinels commanders who led countless battles against the forces of Hell. However, after his son perished in battle, he was driven mad and relentlessly haunted by demonic visions of his son living in a tortured existence. Valen soon fell into desperation to bring his son back to life at the cost of betraying his kin.

Valen made a deal with the Hell Priest, Deag Grav, to resurrect his son in exchange for giving him access to the Wraiths. He gave Grav the keys to the Sepulcher of Elements and opened the vault containing the sleeping Wraiths. The ancient creatures were abducted to Hell and Grav siphoned the Wraiths' power to empower Hell's demonic legions and overwhelm Argent D'Nur, dooming the kingdom to which Valen swore a lifelong oath to protect.

Valen's Night Sentinels participated in a failed and final offensive in the heart of Hell, in which the Order were left scattered across the Unholy Realm and fought to the last man. The treacherous Night Sentinel never did gain back his son; instead, Deag Grav resurrected his son in the form of the Icon of Sin. Extremely guilt-ridden by what he had done, Valen chose to exile himself in Hell. Since then, Valen is condemned by his people as the Betrayer.

Doom Eternal
The Doom Slayer went into Hell, seeking the Betrayer to help provide a battery to power a Celestial Locator, which the Slayer intended on using to track down and kill the Hell Priests in order to destabilize Hell's forces on Earth. Upon meeting the Slayer, the Betrayer warned him that saving humanity would not bring him peace, but only make the burden he carried worse, and that humanity's time had come, much like his people. After providing a battery to the Celestial Locator, the Betrayer also gave the Slayer a special dagger to destroy his son's heart that controlled the Icon of Sin, in the hopes of giving peace to his son's soul. The Slayer carried out the Betrayer's promise, which prevented the Khan Maykr from controlling the Icon of Sin. It is unknown how he lived for millennia but he was probably cursed to live as long as the Icon lives (since the Icon is just neutralized with the Crucible, not dead). He said his spirit will rest when his son's does.

The Ancient Gods - Part Two
Valen's burden seemed to have lightened, as he would later learned about the Icon of Sin's defeat and thus his son's curse has been lifted by the Doom Slayer. He appears as a hologram before the Slayer in the World Spear and gift him his war hammer, the Sentinel Hammer (Hellbreaker). Valen would later heed to the Slayer's call to war on the Dark Lord and rallied the remaining free tribes of Argenta to participate in the final battle at Immora.

Trivia

 * Valen's weapon, the Hellbreaker, is a Super Weapon in The Ancient Gods - Part Two, charged by hitting weak points or performing glory kills. It is noticeably powered by Sentinel energy, likely as a reflection of Valen achieving his atonement.