Citadel (Doom Eternal)

The accursed Citadel is a hellish looking fortress and tower in the center of Nekravol, the City of the Damned. The city and the tower contain Makr civilizations soul processing factory. Within the city's Soul Pits human souls are sorted into those worthy to be processed into Argent Energy, and the rest are discarded into the Blood Swamps.

Background
The factory was built after Khan Makyr made a deal with the Order of the Six. It appears that in this deal Demons and Khan Mayker built the Citadel within Nekravol in order to sort the dead and create Argent Energy for the Maykr.


 * And though our hearts were pure and our purpose righteous, unbeknownst to the fighters of Argenta, in our wake and under the veil of secrecy the Mother God had set her Priests to build vast factories upon the heathen's land that she might gather the Essence for further development. How this deception began we would not know, for our focus remained on the hordes of the underworld. Why then would we stoop so low as to consider our Mother God of betraying our covenant?


 * It was on a mission deep within the boiling seas of the nether realm, where the Order of the Six reside, that a company of Night Sentinels learned of the Khan Maykr's true dealings with the callous beasts. By Maykr design it was kept secret from the Sentinels and Argent people alike that the essence we had grown so reliant on during this war was comprised of the very spirit of our brothers and sisters who'd fallen to these vile creatures. The truth had been uncovered, that all beings who found their end by the demonic horde would become a vassal of the Hellscape, their flesh in time twisted into the very demons we fight, their souls extracted to create the Essence that powers their world...and ours. The victims were the enemy, and the enemy would become us.


 * The soldiers stood in awe at the base of a factory dedicated to such action, a place where the bodies of the slain were sorted and processed, tortured for the mill to have their souls extracted through sinister magick. Enormous vats of pain and suffering, ethereal energy, the haunting sounds of lost souls awash in an eternity of servitude, transformed into energy to power our struggle against the Dark Realm and its malice.

Behind the scenes
Concept art and official lithograph given out by Bethesda refers to this as the "Tower of Babel", see Babel.