Avernum 2: Crystal Souls (http://tinyurl.com/kds5cj5) for Mac OS X is now available at a variety of sources, such as the Mac App Store and Steam. It requires Mac OS X 10.6 or higher and costs US$19.99.
Here’s how the game is described: “Avernum 2: Crystal Souls is an epic, indie, fantasy role-playing adventure in a unique, massive world. The subterranean land of Avernum is full of strange caves, cunning dungeons, and the alien inhabitants of the underworld. Explore a massive, underground nation of tunnels and caverns, packed with over 100 towns and dungeons. Fight to complete up to three distinct game-winning quests.”