The Underworld is an empty and dark area, besides a couple of miscellaneous structures and DaRk MaRt, which contains 1 regular pet machine, 1 premium pet machine (requires Robux to pull), and a shop.
Pulling the regular pet machine requires 25B Tokens, and the premium machine requires 500 Robux. The pets in the Underworld are currently the best-in-slot as of the 13th update (12/5/2018).
Update 13 has added many things to the underworld. Including a 'bad' christmas egg machine that costs 75B tokens to pull, a demon egg pop, and chests.