Urdak also known as Heaven, is the realm of the Maykr race and domain of the Khan Maykr. A hub of transcendent technology, the whole city is powered by a purified form of Argent Energy. According to legends, Urdak is considered to be a living being made of the souls of past Maykrs. Urdak is reminisent of the biblical realm Heaven. However it is questionable if it indeed is Heaven or not. Unlike Hell which truly is Hell it does not appear to be where souls go when they die.
It has been stated that although Urdak looks of angelic origin, it is a technologically-driven domain. Maykrs do not appear to be mortal, but without the stabilizing influence of the Father they inevitably succumb to a state of mental and physical decay known as "Transfiguration." With the Father's influence this would simply result in a voluntary death and rebirth, but without the Maykr instead took to holding back Transfiguration with the rejuvenating power of Argent Energy.
The Maykrs are often seen as divine beings by other races, most notably the culture of the Argent D'Nur. Urdak's name is said to match words for "paradise" or "heaven" in hundreds of different languages throughout the universe, and their iconography have influenced many religions among the races encountered by the Maykrs.
Urdak exists in an anchor state, locked in a static position relative to other dimensions at a sub-quantum level using advanced technology. This essentially inverts Urdak's position in relation to Hell, meaning these planes of existence are fixed outside of the bounds of the known universe, which makes the Maykr refer to them as 'lower' and 'higher' reality. Creatures from Urdak cannot enter Hell, and demons cannot enter Urdak, However after the Icon of Sin is awakened inside of Urdak the holy seal is broken and demons are able to pass into Urdak.
Urdak is the creation of a mysterious being called "The Father." This entity's precise relationship to Urdak is not clear in their history: either splitting into many to form the Maykr themselves, or forming Urdak from himself to birth them. At some point in their history, a rogue Maykr known as the Seraphim betrayed Urdak and stole The Father's essence, which greatly crippled the whole of Urdak.
During the history of Argent D'Nur, the Maykrs contacted the Argenta and offered them an eternal resting place in Urdak in return for their loyalty. Aided by Maykr technology, the Argenta conquered many worlds and built a vast empire to spread the word of the Maykrs, only for the Night Sentinels to discover the true source of the Argent Energy that sustained the Maykrs: vast "Soul Spires" constructed by forced labor in Hell that processed the souls of the innocent. This was a result of a pact between the Khan Maykr and the Dark Lord of Hell, by which the former promised the latter an unlimited number of mortal bodies to be transformed into demons far more quickly than normal, in exchange for the Soul Spires being allowed to operate unmolested.
The Doom Slayer acted against Urdak after the Khan Maykr masterminded the demonic invasion of Earth to harvest human souls for Argent Energy. While the Khan attempted to appeal to the Slayer that this was simply the natural order of things, he fought his way to the Soul Spire in Nekravol, the Citadel, and through the portal at its summit took the battle to the glittering towers of Urdak itself.
Urdak ultimately fell to invasion from Hell due to the Khan Maykr's attempt to awaken the Icon of Sin within the city and the Doom Slayer's interruption of the awakening; The Slayer destroyed the Icon of Sin's heart, which would have allowed the Khan Maykr to contain and control the demon. The Titan broke free, unsealing the barrier between Urdak and Hell, and the Slayer killed the Khan Maykr herself in combat. The demons of Hell immediately ended their pact with the Maykrs and invaded Urdak.
It is implied, though never stated directly, that Samuel Hayden either is the Seraphim Samur Maykr or is related to him in some way. The AI VEGA, on being connected to Urdak's systems, questions if he is actually the Father, though this question is not answered.