Small World Underground is a standalone game set in the underground world of Small World. In this area control game, players must fight for survival and points, using 15 new fantasy races and 21 unique special powers to take over the underground world. Each race can be combined with any of the new powers, allowing for endless combinations and strategies.

The 15 new fantasy races in Small World Underground include Cultists, Drow, Flames, Gnomes, Mudmen, Mummies, Ogres, Shadow Mimes, Iron Dwarves, Kraken, Liches, Lizardmen, Shrooms, Spiderines, and Will-o'-wisps. Each race has its own strengths and weaknesses, and can be combined with a variety of special powers to create unique and powerful combinations. Some examples of the new special powers include adventurous, which allows players to collect an extra victory point for every "popular place" they occupy; immortal, which allows players to redeploy all of their race tokens even when attacked; and vampire, which allows players to replace active opponent's tokens with their own.

During their turn, players will play race tokens to control and combat over areas of the underground. If they are attacked, they will lose the area and must discard a race token, but can redeploy any others that were removed. Eventually, a player will not have enough race tokens to deploy and must decline their race and choose a new combination on their next turn.

In addition to the new races and special powers, Small World Underground also adds new tokens to the game, including popular places, a volcano, and righteous relics. These tokens add new challenges and opportunities for players to navigate as they compete to conquer the underground world.

The game is designed for 2-5 players and takes approximately 90 minutes to play. It is recommended for ages 8 and up. Small World Underground is a standalone game, but is also compatible with other expansions in the Small World series, allowing players to mix and match to create their own unique gameplay experiences.

Overall, Small World Underground is a fun and exciting addition to the Small World series, offering new challenges and strategies for fans of the game to enjoy. Whether you're a seasoned player looking for a fresh twist on the game, or a newcomer looking to dive into the world of Small World, this expansion is sure to provide hours of entertainment.