What it adds is, as the name rather implies, Exar Kun's tomb to the game. It's a bit outdated compared to some of today's new area mods, (in that it re-uses an area) but it's still worth exploring. Most of it is a hackfest through the various Dark Jedi guardians (hurrah for more XP), but at the end your character will be confronted by Exar Kun's ghost and have the opportunity for an alignment shift as well as some good loot.
To reach the tomb visit the Yavin space station, and then go back to the Hawk. At that point you'll be presented with an option to travel to Yavin IV, though if you don't have a savegame handy you can always warp to the module directly. (The name being 'ekun'.)
Well, that's that. Enjoy!
-Emperor Devon