I am pleased to announce that the 1Quest Modding Document is finished! You'll find it at
http://ratzngodz.fr/wiki.
Be sure to start by the first examples in the "Getting Started Section". It will give you a valuable first experience in editing the xml files of 1Quest.
Modding can make you add/remove/modify spells, monsters, items, interface, levels, even combat rules, magic system or experience system ... about everything. You can even build a entirely new roguelike!
The documentation will continue to evolve as I clarify some part. Feel free to report anything you do not understand so I can focus my effort on those pages, or add new ones.