Ragnarok

Ragnarok is a freeware roguelike video game also going by the name Valhalla in its European release. It is distinct from many other roguelikes in having a graphical interface, a historical/mythological setting, set quests, the ability to change classes, and the ability to permanently change one's race by polymorphing. Source: Wikipedia, "Ragnarok (video game)", available under the CC-BY-SA License.