|doc the grey|
You could put a robot as the guardian, a warden robot would be perfect fit. Quite tough enemy, especially if the group does not have any admantine weapons.
What excatly would it be left to guard?
I suggest a computer terminal that might or might not be intact. What secrets does it hold?
Or maybe a tech lab that savvy PC's could use to craft tech items or even robots of their own?
The Technology Guide has lots of useful stuff. Check it out on PRD.
The Warden is pretty awesome but considering the confines, the difficulty of getting back out of this space, and the powers he has I might want something a little lower on the CR curve. Otherwise I might have them drop in, hit that aura, and get mopped up. But he's still really f$$!ing cool and thematic and it's tough saying no to him lol.
As for your other questions of what this creature is guarding the pitch goes something like this. The room that this creature occupies is one of the few rooms that has remained largely intact, all of its walls and security doors either remaining intact or having been replaced with the sheer rock walls of the cavern itself. This has left the only real way into the space as a large hole in the domed roof of the room which can only be accessed by scaling the rock walls above and then descending down into the space. This wasn't really an issue for the previous occupants and the Morlocks to their credit thought of the room as a sacred place of worship. From here the Morlocks would clamber down from above to the lip of the roof, offer prayers, and then dump offerings into the room below. Believing it to be sacred they never actually traversed into the space themselves. The space is meant to be reminiscent of the mayan Cenotes, where the morlocks cast objects and sometimes creatures into this "tech well" to appease the "spirits of steel" their faith worships. With that in mind the creature in their could be living as their would likely be enough organic material dropped into the room to sustain something.
Hope that helps people get more ideas.