Gehenna is a location found within Shadow Man 2econd Coming. It is the Grigori's stronghold and acts as the hub of the entire game, comprising of The Pit, Sigil Chambers and the Dark Sepulchre which house the various Liveside Levels that Shadow Man must hunt down in order to destroy the Grigori.
The entrances to the worlds are protected by the remainder of Asmodeus' generals. In the center chamber is The Pit. From here, Shadow Man can reach Dark Sepulchres around the world. Two areas are inaccessible which likely belonged to Sammael and Thomas Deacon or possibly the cut Grigori Djinn. The area is decorated with Nephilim fire which bathe the level in an infernal red light, as well as various demonic symbols. Armoured-covered bones can be found throughout, implying previous failed expeditions over millennia.
The area acts as the location of the final confrontation between Shadow Man and Asmodeus.
(from Shadow Man 2econd Coming Game Scriptment and Strategy Guide)