That building was originally a military facility that was then turned into offices. But the elevator could still go down to level -20 where there is a hidden base during cold war they used to do military research. For some reason, the elevator takes you down to that level and you get out to go home after work. But once you step out the elevator you saw smear writing all over the walls with blood. “Naruka..naruka..” all written on the dark halls. After family notice you missing,They immediately send a team down trying to find you but they never find you. They found parts of your shredded dress but no other sign of you. they permanently took down the elevator and sealed it with concrete to stop anymore accidents. Apparently military was using that facility to test mutations caused by nuclear radiation before they built an office on top. Deep in the darkness you lay unconscious hiding under a stacked table behind a wall