Back in the days of Quake editing there were many partial and total conversions. Many were hardly worth playing, but there were several good ones. A popular setting was the medieval-fantasy type style. Unfortunetely, there have really been no big total conversions for Quake2... except the Dawn of Darkness! However, the team working on this mod had alot of problems with it and, after dropping half of the game, just recently released the first episode.
Dawn Of Darkness takes place in a long vanished world of mighty forgotten empires and magic. It is the tale of a once revered Lucitanian warlord, Roarke The Merciless, caught between his obligations to his family, his empire and his honor.
The gameplay was good, with all new weapons and monsters, but the downside is that sometimes you'll run up to an enemy and hit fire and nothing will happen! Also they can deflect your attacks with their shield. The new monsters and weapons looked great though.
It is also quite a puzzle solving game. And with many different routes (it's very non-linear) it can get a little confusing.
The good part is level design... it is amazing! These levels are the most atmospheric I have seen in Quake2. From little villages to castles, caves and swamps. I especially loved the terrain, it is easily the best constructed for Quake2 I have seen. Some of the level designers which you may have heard of from their Q2 maps are Ben Glover (Homage To Mars), Brendon Chung (Citizen Abel) and Jerk Gustafsson (Age of Panic, 1964).
Personally I don't like puzzles and too non-linear, so this wasn't the mod for me. But I am sure there are gamers who will love it. It is only one episode with four huge levels (one of them is 7mb, easily the biggest level ever made for Quake2) and it will definitely keep you occupied and entertained for a while.