(Iter Vehemens ad Necem)
From the author's description:
"Fellow adventurer, turn back while you can! For here begins the roguelike Iter Vehemens ad Necem, a Violent Road to Death. If you choose to travel along it, you will dive into countless exciting adventures to gain items of great magic, attain powerful equipment made of mysterious materials, bathe in the blessings of mighty gods and recruit loyal allies of various shapes and sizes. Unfortunately, along the way you will also often be dangerously injured, poisoned, catch numerous diseases, lose several limbs and transform into manifold different kinds of pitiful creatures in the darkest depths of hostile dungeons. And, at the end of the road, you are bound to perish in a most gruesome and painful way. Don't say we didn't warn you."
The unoffical IVAN wiki has a lot of information about the game, including some "spoiler" pages on the gods and monsters.
A post on the old IVAN forums details some of the bosses.