The Titan refers to an ancient demonic entity, of enormous size and strength, born from the bowels of Hell, created by the dark lords to defeat the Doom Slayer. Their plan failed, and in an unknown age the Titan was felled by the Slayer. The Titan was one of the largest demons to ever exist, second to the Icon of Sin.
In response to the ever-growing threat of the Slayer, the Lords of Hell decided to create a demon strong and large enough to defeat him, and thus the Titan was born, and soon he faced the Slayer. The Titan drew strength from the countless demons killed by the Slayer and a mighty battle ensued upon the Umbral Plains. Both sides fought for some time, until the Titan was killed and, unlike other demons, its corpse did not burn away and remained embedded into the Umbral Plains. It is unknown how this Titan was killed, but it was likely torn apart by the Slayer with an Atlan.
The corpse of the Titan still remain in the Great Steppe, and non-significant amount of deterioration has occurred, and demon kind has used the body to build temples and rooms into the beast's internal structure. It is presumed the Titan's remains will remain in the Hellscape until the end of time.