Well,people already said pretty much everything you need to know in terms of why you should play this,but there is one more thing: mods! This game had some decent modders working on it back at the day.And they produced all sorts of good stuff,like bunch of sprawling adventure maps,comparable to Dark Souls levels,maps with interesting game modes, like "hold the fort" or the PvP-PvE arena with monsters as playable characters included,and more.
