|This message greets you when you zone into the "Gnolls' Den":
"Sounds of chanting and blasting magic echo through the hallways. The patter of footsteps grows louder as they seem to approach you."
Throughout the dungeon are Magical Portals, you will need to destroy these objects to stop the mobs from spawning and attacking. To quote Meri of Bedevere, "Wait too long and you will be overrun with the denziens birthed from these vortexes."
Here is a map provided by Meri of Bedevere showing the portal locations: