Titans or the Ancestrals are mighty beasts of Hell that are natived from the realm Argent D'Nur, far larger than regular classes. They serve as beasts of burden for the more powerful demonic entities, and as combatants in the armies of Hell.
During the war between the Night Sentinels and Hell's forces, Titans appear to have been Hell's counter to Sentinel Atlans, though they do not seem to have fared particularly well against them. Titans do not seem to have been deployed to Earth on the same scale as they were deployed to Argent D'Nur, with the beasts seemingly only used in a rear-echelon role to transport the great temples of the Hell Priests.
They are implied to be enormously difficult to kill, with only Altan weaponry or the hellish blade of a Crucible shown as able to kill one: in the latter case, not permanently, as Samuel Hayden states that even if they are impaled through the heart, if the Crucible blade is ever removed the Titan will be restored to life.
A single Titan, seemingly a particularly large or powerful one, faced the Doom Slayer as he rampaged through Hell, but even this mighty beast proved no match for him. The precise manner in which he defeated the beast is unclear, but its skeletal remains are strewn across the area of Hell's Great Steppe known as the Titan's Realm.
Living Titans are present in the Hell city of Nekravol, with one chained in a pit of lava outside the demonic city's gates with streams of molten material pouring from its eyes, and two more present at the highest level, pulling on chains to hold a section of the Maykr energy transfer device in place.
Another unique Titan, the Icon of Sin, was birthed from the torture of the son of the Betrayer and controlled using his heart. Later the beast was transported to Urdak and modified with Maykr technology to allow the Khan Maykr influence over it, but the Slayer destroyed the heart and left the beast to its own savage instincts, ultimately planting a Crucible blade in its exposed brain and ending the demonic assault on Earth.