You grip the sides of the hole, sliding forward so that your legs dangle over the edge. Then you shift yourself forward farther, letting yourself slip over the edge so that you're hanging by your fingers from it. Your hands burn from the effort, but you grip grimly on, refusing to let yourself fall too soon.
Then you let go. It's a short drop down, all things considered, though it still takes you a moment to regain your balance.
This new tunnel is narrow, the walls closer together and the ceiling far lower. You don't quite have to stoop, but you'll not be attempting a new high-jump record down here. There are no torches on the wall, but there's a still light: a dull, dim, crimson glow. It doesn't appear to be coming from anywhere; it's just…here. Everywhere. Without any source.
You move slowly forward. The passageway that you're in has a sharp corner up ahead, and quietly as possible, you move up to the edge of it, peering around.
The corridor doesn't go on much farther: maybe thirty feet separate you from its end. The end itself is an ornate door, looking as though it was formed out of the same rock that makes the walls and ceilings of the passageway. Carvings cover it, depicting humans and the undead alike suffering despair, torture, and gruesome deaths. As an obstacle, it looks formidable, though nothing like as formidable as the creature that stands in front of it.
It's no demon, or at least not a kind that you've ever seen before. There are no scales or long claws, and it lacks the graceless, bowlegged appearance of the fiends you've encountered until now. Instead, the figure has an almost elegant appearance: a slim, humanoid body wearing what appears to be white silk, with long limbs and a calm, pale face. If it wasn't for where you'd found him, you could kid yourself that this was just another mortal.
You gaze at the creature, who doesn't seem to have noticed your presence yet. Or, if it has, has decided not to respond to it. From what you've learned about the undead and the realms beyond your own, your best guess is that this is some form of lieutenant: a being with more command among the legions than the ones you've encountered so far.
It seems likely that whatever you need, you're going to need to pass through the door to get it. How you're going to get through it, and how exactly you're going to be able to get past its guardian, is something you still need to decide.