This mod adds a dungeon outside of vault 101. required dlc the pitt.
Story

In ancient times there used to be people who worshiped evil gods. They used to sacrifice 60 people a year.
One day a young man amongst them betrayed their gods and the wrath of the gods hit their city.
Their city sinked deep into the ground and went to sleep, a sleep that lasted more than ten-thousand years...
When the bombs struck DC the ground plates were disrupted and thus, the ancient stratum arose, and now you see their
temple right in front of vault 101.
You may enter the temple... But you may not come back....