I sure hope my video card can handle this...
Combat in Avernum is turn based, with faster characters moving ahead of slower ones. When entering combat, the party is transported to a battle screen. Each turn, a character has a certain number of Action Points they can use. Moving 1 space uses 1 AP, and actions also use different amounts of AP. There is a bit of leeway with characters, since they must first be reduced to 0 HP and then take another hit after that before they are killed. Monster groups are not pushovers, and it's not a good idea to stray too far from town without either good equipment or a good healer to back you up.
To this day, there are 5 games in the Avernum series. Avernum 1-4 follow the main plot, with 1-3 being remakes of the original three Exile games. The fifth game, known as Blades of Avernum follows the same concept as another older game, Blades of Exile (see a theme here?) where different modules could be loaded, allowing for different adventures. This version also comes with a construction kit, allowing people to create their own modules using the Avernum engine, though I haven't looked into that so much. Avernum 5 is reportedly in the works.
The games are shareware, and are still sold by the original creator, over at http://www.spiderwebsoftware.com
, although there are demos of each for download. Registering gets you an unlock code to access the rest of the game. (I'm sure there are unscrupulous ways to get these codes out there on the interwebs, but hey, if you like a game, why not support it?)