Underdark Adventure Advice please.

[MasterMercury]

Hey guys. I've been planning this particular adventure for a long time, and now my players are finally on track to get to it.
The plot is that they are using an underground "shortcut" to sneak into a hobgoblin fortress to rescue the son of a noble woman.
The entrance to the passage is in a hob fortress that they already cleared.

They go down, and a carrion crawler runs on the ceiling in their direction. If the party doesn't attack, they're fine.

After that, they hit a bump. The passage has been split in two, with a huge chasm between the two tunnel entrances. They see a stone "bridge" beneath them leading into a fungus lit tunnel. The floors and walls of the tunnel are immaculate....

A gelatinous cube will be following them for an hour, slowly silently trudging behind them. Hopefully, the party doesn't notice until.... they come across a Quaggoth (underground Bigfoot) feasting on a carrion crawler.

Now they are trapped between a monster and a wall of acid.

Advice needed: How can I foreshadow the cube so it doesn't seem unfair that it just appears. Any other combat suggestions? I have a part 2, but I'll save that till they complete this. Thanks in advance.

[Arcangel4774]

I assume the immaculate nature of the cave is due to the acid wearing everything smooth. Maybe have slight divots or irregulaties that farther into the walls than the smoothed part with half corroded this-or-that within it.

[DMThac0]

Describing this "immaculate" tunnel in detail would be my approach. Consider what it looks like when acids erode materials; the walls have striations in them like someone used finger paints on the walls, the ground is smooth in appearance however you can't help but notice there are sweeping ruts in the floor like someone drug a heavy object through clay. There's a slight acrid smell in the air no matter where you are in the tunnel, with the lack of air movement it hangs like a miasma just on the edge of your senses. Give some variations in depth of the stone showing how the less dense stone was eaten away where as the more dense/less susceptible materials are left behind.

Before they head into the cube's lair use the same kind of overly descriptive approach to describe how lively the tunnels are. The fungus living in each of the crevices, the lichen clinging to various rock faces, the roots or tendrils of other plants winding through cracks in he walls. Small bugs skittering around on the ground an walls, tiny creatures like slugs lounging around and lizards quickly darting about as the party's light scares them off. The air is musty, almost stale, due to the lack of air moving around as much.

[MasterMercury]

Awesome, thanks guys. Very helpful