Dark Lord

Dark Lord is the title of the supreme ruler of the realm of Hell. The title appears to refer to any entity which claims dominion over Hell, with the realm's past being divided into "ages" based on the reign of these creatures.

Davoth
The original Dark Lord was an immortal being once known as Davoth, created by The Father to rule over his first deliberate creation, a realm called Jekkad. Jekkad was a higher plane than even The Father's first creation, Urdak, and Davoth was a Primeval, one of the first gods, created to rule over it.

The people of Jekkad made many great wonders, seeking to make their land a paradise: The Father had filled them with ambition and granted them a lack of restraint, wishing to see them excel in all things. They were long-lived, but not immortal like the Maykr of Urdak, and it was in this single flaw that the seed of Hell itself was planted.

The Father created Davoth with an undying love for his people and a desire to better their lot, and both he and the denizens of his realm came to regard mortality as a curse. Davoth knew that he himself was not mortal, and feared the possibility of his beloved people's extinction, leaving him alone in an empty realm. Turning their intellects to defeating death itself, and without self-doubt to reign them in, the people of Jekkad, guided by Davoth, commenced their search.

The Father saw the future of this endeavour, that Davoth and his people would never cease in their search, regardless of the cost. Foreseeing that someday this ambition would threaten all of creation and that Davoth would someday seek to challenge The Father himself, The Father sealed off Jekkad from all other realms.

Davoth saw this act as an unforgivable betrayal by his creator, directing his fury at the barriers that encircled his domain as his people continued to fall to mortality. He became The Father's nemesis, the Dark Lord, while his realm would come to be known as "Hell" to the people of Urdak. The once grand and beautiful realm fell into horror and ruin as its denizens set aside all other concerns save for their singular purpose.

Davoth continued to test at the boundaries of his prison, with his legions spilling forth into other realms, defeating many forces sent by The Father to stop them. With each victory, the Primeval's power grew greater. Finally moved to act, The Father entered the despoiled realm and, atop the Pyramid of the Lost, once a temple to The Father, he tore the essence from the Dark Lord's body and cast it down. While Samur Maykr begged The Father to destroy the Life Sphere containing Davoth's essence, he refused, as Davoth was still his own creation: instead, he placed the Sphere in the Temple of Souls.

While Davoth was reduced to nothing but a sphere containing his essence, he continued to whisper his influence to the corrupted people of his realm. Ultimately, he would be restored to a physical body by the Doom Slayer through the use of the Maykr device known as the Luminarium, as part of the latter's plan to destroy the Primeval and in doing so Hell itself. It is unknown why the Slayer didn't simply destroy his Life Sphere. When resurrected, he appeared in the copied form of the Slayer.

Later Dark Lords
Following Davoth's near-destruction it appears that other powerful demons within Hell vied for his former rank and appointed themselves as Dark Lord in his place.

Given the chronology of events, it appears to have been Davoth's successor, the Dark Lord of the First Age, who made a pact with the Khan Maykr allowing for the construction of "Soul Spires" in Hell. These structures, including the Citadel located in the Hell-city of Nekravol, fused mortal souls with Hell energy to form Argent Energy, an incredible power source which the Maykr could use to stave off their degradation following the loss of The Father. In return for allowing the construction of the towers, Hell would be provided new worlds to harvest, as well as the refuse from the soul extraction process, husk-bodies which would transform into demons much more quickly than if the process proceeded normally.

A voice identified as the current Dark Lord (presumably of the Fourth Age ) was heard contacting Olivia Pierce. This is implied to have been the Spider Mastermind, which was later destroyed by the Doom Slayer.

Trivia

 * The voice that calls out "NOOOOOO!" after the death of the Khan Maykr in Doom Eternal is identified as "Dark Lord" in the game files, but was originally only identified as "Mysterious Voice" in the game itself. After the release of Ancient Gods, this was changed to match the game files.
 * It is possible that the Slayer is the parallel universe counterpart of Davoth since Jekkad was a regular realm before. That's why they look identical.