Shelob is a special Spider NPC, there is no known difference in psychical appearance, however Shelob has about twice the health than a regular spider. Shelob can be found wandering the swamp levels, or inside of a small room with spike trap floors inside of the mines. (Pulling 1 lever will trigger all the traps, pulling the second lever will open the gate)
Shelob will drop Dyrnwyn, the magic sword, as well as a -5 cursed ring of invisibility.
TriviaThe name Shelob is a reference to the great spider from the Lord of the Rings. The -5 cursed ring of invisibility is another reference, a cursed ring that turns you invisible mirrors "The One Ring to rule them all"