Actually it's more of a roguelike, but it has a story and some other more RPGish elements. Also some rudimentary but fitting graphics (not ASCII)I've played this game probably around ~300 hours and have yet to make it past level 30 (normally you'll end up 45-50 to finish, though I've never seen the ending) It's very challenging, but very fair and very intuitive too. It has a great tutorial that explains the fairly complex gameplay well, and has tooltips for practically everything, and a just a great interface all around. Frankly it has one of the best interfaces I've seen in any game. It also has a built in chat in-game which is helpful so you can ask questions without having to go to the forums.
It's hard to explain why it's so fun, but it's a combination of really satisfying turn based combat, "loot drop", and an enormous variety of monsters to slay, weapons to loot, and classes, skills and spells to choose from. Combine that with permadeath which I think makes any gaming experience much more edge-of-your seat and it's just one of the best games I've ever played. (though the "standard" mode gives you about 7 lives over 50 levels spread out, even with that though you're unlikely to make it out of even the beginning dungeons your first few outings)
The class design is really one of the high points though. "Solipsists" that can invade an enemy's dreams and attack their mind directly (basically it throws you into a battle in said enemy's subconscious), chronomancers that can manipulate the flow of time (among other things you can split a time thread and then choose what result you want after playing each thread separately), shadow blades that can phase through walls and animate their own shadows to do their killing for them. Anyways, a lot of very creative stuff. In all there's something like 20 classes total.T
There's also a total absence of grinding and potion hoarding and the atmosphere is well done with lots of interesting, well written lore tidbits lying around that you can find.Anyways, check it out if you enjoy tactical turn based games or RPGs. Or just something that's really difficult. It's free and just got its official 1.00 release.
Worth mentioning too that the one man dev team is a fanatic who's released something like 5 updates in the past 2 weeks. The community as a whole is probably one the nicest and most helpful I've seen.