A Lair in the Underworld

In a netwoek of tunnels underneath Parisius is a delapidated entrance, and not just any entrance. Some have speculated this entrance is an opening to an underworld of localised evil. Whispered to have been there for millennia since the early days of the local Celtic Parisii tribe or even earlier, it is believed to contain the essence of a localised threat that terrorised them.
It was built upon when the Romans arrived without them knowing that such a creature existed, modifying it when they themselves realised what it contained. Then after time when the Franks arrived they decided to build the Notre Dame Cathedral over it to help contain it.
As the myth goes, deep below the grounds here an abomination from when the world was young was birthed in a cave deep under the ground here. It's parentage was that of ancient early deities that lived on blood, flesh and bone. Horribly nutated they appeared to those who saw them as a gibbering mass of gelatenous limbs, tentacles, mouths, and eyes popping out of a blubbery mass of tissue for a body. When the young abomination was born, the parents roamed far and wide taiking all manner of creature to their lair to feed their young.
Then the Parisii arrived. Drawn to small caves that were carved out from the rock & soil of the Seine, they explored the caves chancing on this lair. The parents were killed at a heavy cost to the tribe, but with the help of the local druids they sealed the passageway that acted as the lair. One thing they did notice was that the passageway went slightly deeper and seemed to end at an entrance to somewhere else, somewhere not of this world. Sealing it helped to prevent anything else coming through. Or so they thought.
For centuries they felt safe from this abomination. Then the Romans came, who donimated them and started to carry out underground waterways and hweating for their buildings. Finding the passageway they opened it after finding it was empty behind the seal. Doing so they released this mass of hungry flesh & blubber and suffered in general. Quickly resealing it and spiritually & magically locking it further still, they again stopped more carnage happening.
Ever since this time and the records being stored in the local Frankish nobility, the place remains sealed and protected more. Notre Dame Cathedral was build over it to add further protection.
Unfortunately, scratcing and strange squeels can be heard coming from it at times when in the vicinitiy. Sometimes the earth, rocks and cement seems to split open, usually around the time of Samhein and visions appear overlaying the opening, one of arms & hands coming out of the ground & walls reaching for something in the darkness beyond.
Any that have studied this claim that it is the damned, the victims of the creature, and/or worshippers who were sacrificed to it.
As for the latter, there does seem to be the remains of worship scattered around the entrance and carved into the walls. Layer upon layer of these have built up over the centuries stretching back to Neolithic times. Pirimitive religious items remain there with the odd ancient bones. What annoys the local authorities more is there appears to be recent activity around it as if recent cult activity has took place.
So far nothing has came of it, though the Parisian authorities have tasked the Thieves of Frankia t ohave keep a guard nearby in the tunnels & warrens around it. The Thieves have generally avoided it over the years as they constantly report strange occurances around the area. They have took the job up begrudgingly, expecting to be well-compensated in jobs, gold, and the local militia to turn a blind eye to more of their jobs in the city overall.
They have been reporting back thes odd occurances and many report the feeling of being touched or grabbed in the area. They also say that shadowy figures are seen on more than one occasion.