Island in the Mist
The vast, connected labyrinth of caverns and man-made passages beneath the Mists. Long ago, powerful wizards built a multitude of mysterious portals, numbering in the millions. These portals lead all over the Mists, possibly even to other planes (unconfirmed speculation). Even further below the Underworld lies a place not unlike Hell itself, full of malicious demons constantly seeking access to the Mists.
The number of dangers the average person faces in the Underworld are breathtaking. Many plants are outright hostile, most are poisonous. Sparkling diamonds are actually small disintegration spells. Most denizens are vulnerable to silver, which is in scarce supply here.