Disclaimer: i was one of the playtesters for Underworld Stories but have no formal association with the game or its author, e.g. no financial ties.
Underworld Stories is an solid, ultra-light solo dungeon crawl game which plays in less than half an hour. It's conceptually similar to Four Against Darkness, but (A) the rules are both much lighter and much tighter, and (B) the player controls but a single character with no choice of class, race, etc. (That said, the game engine is flexible enough to easily customize it for new character types.)
For randomness it uses a deck of 18 cards - the exact same deck used by the designer's "Hannibal ad Portas" game and other upcoming games in his 1-Page Game series. The cards depict many types of information, each of which is used in different contexts in the game, e.g. generating dungeon layout, populating rooms, attribute tests, combat resolution, or even NPC/monster reactions.
Printed out, the tabletop footprint is 2 pieces of paper: one for the character record sheet and one for the dungeon. The game includes several dungeons, each with distinct monsters and treasures, and completing them grants the character XP with which buy new abilities or improve attributes for future delves. The character sheet contains everything one needs to track the character's state and includes a summary of all important rules of the game. After only one or two sessions, referring to the rulebook is no longer necessary. The dungeon page contains all tables required for that particular dungeon and has an empty area for drawing out the dungeon (most of which have random layouts determined by the cards, though fixed-layout dungeons are also possible). My recommendation is: rather than print out a new copy of each dungeon for each delve, put the pages in clear binders and use a dry-erase marker on them.
The game's environment is completely generic, including no "flavor text" or "world background", so if background/flavor is important to you, this game will not be a great match for you. If, however, you simply enjoy roaming dungeon halls, slaying monsters, and taking their loot, without any significant bookkeeping, then you'll be right at home here. Likewise, it may also be of interest to players who enjoy seeing new and clever game mechanics.