Very nice and polished traditional roguelike. A bit too easy for my tastes, though - some levels can be crossed without encountering a single demon, the cooldown on your banishing spell increases rather slowly, there are plenty of wards and potions of refreshment, plus demons never come in groups so you can easily retreat until you get your instakill attack off the cooldown. The only truly dangerous demons are the archfiends, who on the other hand have a very, very nice attack!
But take this with a grain of salt - I think that any game that doesn't kill me several times before I learn how to get at least into mid-game is too easy. :) I won this game on the first try, and if the dev wanted to make a post-challenge release, I would love to play it again with spawn rates increased and ideally with even more demons spawned immediately after picking up the Star Ruby.