Game description from the publisher:

Talisman: The Deep Realms Expansion allows you to travel between the Dungeon and the City regions by venturing into the dark, dank, and haunted passages that connect them. You can cross through the two corpse-filled and rat-infested Deep Realms by means of a perilous bridge or venture even deeper into an ancient system of tunnels inhabited by rats, feral pets, and much worse monsters. In the gloomy and perilous Deep Realms you will certainly encounter tremendous danger, but you may also find tantalizing wealth or even a Talisman.

To use the Deep Realms expansion, you need the Talisman base game as well as both The Dungeon and The City expansions.