Titan

Titans are gigantic demons, first mentioned in the lore of Doom (2016), and appearing in Doom Eternal. Despite their incredible size, most Titans are not enemies in the game; they are instead referred to as thralls, mere beasts of burden for the forces of Hell.

Hell barges
The Hell barge is a class of Titan used as transportation, carrying massive portable temples. The first Titan encountered in the game is such, and is seen in Hell on Earth after killing the Hell priest Deag Nilox. It wanders away aimlessly, the psionic backlash of its master's death having apparently rendered it mindless. Other Hell barges can be seen during the course of the level carrying temples, presumably transporting the other Hell priests to the Citadel for the Deagic Council.

Other
The second type of Titan is seen in Taras Nabad. It is already slain, said to have been defeated by the Doom Slayer long ago. The hilt of the Crucible is retrieved from its heart.

Other enthralled Titans are seen in Nekravol. Notably, in Nekravol - Part II, two of them hold the Argent transmitter tower aloft on chains. Destroying these chains allows the destruction of the transmitter, via which the Doom Slayer can reach Urdak.

The last Titan is the Icon of Sin, met in Final Sin. This is the only Titan that is actually fought during the game.