The fight to get down into the bunker had been arduous, with hundreds of demons roaming around that seemed to have nothing better to do than try and take the Doom Slayer out. Even though they mostly only fought with tooth and claw against the slayers' guns, it had still been a hard fight. Each demon could take enough punishment that if the Slayer wasn't careful, they would get up close and personal. By the time you had reached the main bunker, your armour was already pierced in several places, with sections missing, showing off more of your skin, which seemed to drive the demons even more wild.
At first there hadn't been any sign at all of the human colonists of the place, but as you descended down into the bunker, you saw them. Luckily, most of them were just dead, killed in some ritual or other fodder for the demonic armies, oddly you didn't see any females among the bodies used for the rituals. Had they been taken off world for something else instead?
The deeper you went, the hotter it was getting and the larger the demons seemed to be, taking more damage to finally put them down. The last one to eat your shotgun just as you entered the vault managed to tear off the crotch of your armour before having its head blown off. In front of you among the corpses of the demons was something very different. the technology of the demons wasn't so much mechanical as some kind of biological machine; blood-red runes formed out of blood-red slimy tendrils that pulsed in time with a heartbeat.
The echo of your final gunshot faded, and what you had thought was a wall in front of you shifted, ripping as it did before slowly pulling back. Shockingly, you realised that it hadn't been a wall; no, that was the wing of a monstrously sized version of the demons you had been killing on your way down here. As the wing folded back, you finally saw it, or rather, you saw its leg. The being before you was so big that you could have walked between its legs and barely had your helmet brush against its thighs. The hum of machines was the computers from the colonists that had all been combined into the network of bloody cables surrounding the primordial being; several of those machines were 'chanting' a ritual being played out using the speakers of the machine, and guttural screams and words that you didn't think you wanted to understand filled the air. A deep rumbling voice was echoing, and yet it sounded so weak too; it was clear that whatever was happening to this monstrously sized demon was actually hurting it.