Going by the murals written in the last room, you wonder if the undead had something to do with it.
@Both of you: Anyway, as soon as Sovelin lights the torch for his non-darkvision human friend you hear something screech from above you. Looking about you see that you have stumbled into some sort of giant spider's web!
What you thought to be a giant sack is actually a cacoon, probably the spider's last victim. Looking above you, you see an enormous, sickly, pale green coloured spider descending from the ceiling.
Please roll initiative and take your first turns.