@Alkaid: Confirming what you already know, you realize that the altar is to Lamashtu.
Pushing open the doors, you enter into a huge room, with a 20' high ceiling, looking like nothing more than an immense underground cathedral. The walls of the chamber are carved with strange, spiky runes, resembling those in the odd spherical room. In the center of the room is a large pool, with a ring of polished human skulls balanced on stone spikes arranged in a circle around the deeper midsection. At the far end of the room a pair of stone stairways leads up to a pulpit on which sits a second pool, this one triangular in shape and filled with churning, bubbling water that looks almost like translucent lava. Yet, while wisps of what looks like heat and steam rise from the strange orange liquid, the room itself is deathly cold.
The stone doors to either side of the entry hang open, revealing seemingly empty rooms beyond.
You have more important things to worry about for now, though.
On the upper level, next to the glowing orange pool is a strange, thin, wiry, winged female creature with ram horns curling back from her twisted head.
She shrieks in rage as you enter. "Defilers! How dare you intrude upon the Mother's sanctum?"
She then slashes her own wrist with a dagger, allowing blood to drip into the bubbling orange pool. As it splashes down the glow fades somewhat and a creature, resembling those you've already fought, springs from the pool and moves to attack.
The female creature seems surprised and upset at the fading of the glow.