The survivors have reached the end of the cavernous subterranean passage...

Mongo: What now?
Ged: It can't just end.
Belphanior: Why not? No telling whether anyone built with any coherent sense.
Ged: Yea, true.
Peldor: (points off to one side) Look there.

The thief had spotted a large, oval side passage, the opening nearly covered in thick goo that hung from the area overhead.

Lyra: Ugh. Do we have to go through that?
Ged: We don't have to, but it's the only way to continue.

Suddenly, they heard hissing and scrabbling sounds from within the side passage...and whatever lurked within was coming closer!

Belphanior: Here's a better idea: let's blast it with a fireball first, then go through later.
Ged: Yea, good plan.
Alindyar: Agreed-

The debate was broken up by a high-pitched screeching sound, as one of the weird monsters - a full-sized one, at that - leapt from within the side area, bursting through the slime to land amidst the party!

monster: Screeeeee!

The thing had obviously been hiding and planning its attack, for it struck with lethal speed. A clawed arm knocked Mongo aside, leaving Alindyar and Lyra a mere ten feet from the attacker, without armor or weapons to protect themselves.
Otto, however, was on top of things.

Otto: Take this, you bastard. (he fires a crossbow bolt at rather close range, hitting the thing in the head)
monster: (recoils) Screeee!
Alindyar: (pointing both hands at the thing, he unleashes a sheet of flame, driving the thing back in the direction from which it came)
Ged: And if you liked that, you'll love this. (he sends a fireball right past the monster)
Peldor: You missed.
Ged: No, I didn't.

The spell exploded somewhere within that side passage, incinerating whatever might lie in wait back there and sending a great gout of flame into the main tunnel. The monster, standing right at that area's opening, was roasted instantly, falling in a blazing mass of melting flesh. Due to the party's position, they weren't hit by the flames.

Belphanior: (eyeing the fiery passage, even as the flames die down) Nice.
Ged: Of course.
Alindyar: What of Mongo?
Mongo: (lying on the floor about twenty feet away, he groans)
Lyra: He looks like he's okay.
Peldor: A fourth monster...
Songa: Not anymore.
Otto: So that's four dead, now.
Ged: Which should be all of them.
Belphanior: Unless there are more that we don't know about...

There was a violent earth tremor then, the floor beneath their feet shaking and lurching.

Ged: Boccob!
Peldor: What the hell was that?!?
Belphanior: (hearing sounds from the still-smoking passage) Uh...I think something's going on in there.
Ged: What do you mean?
Belphanior: (aims a wand at the passage) I'm not sure, but it can't be good.
Alindyar: (preparing a spell)
Otto: Sounds like...

Just then, a huge crack appeared in the wall of the main passage, smaller cracks branching off from it. All of these fissures in the structure of the underground chamber were widening, which along with the shaking of the cavern floor, meant one thing.

Otto: ...something's coming through.