Hot off the PDF presses, I'd like to present to you Descent the RPG, an add-on expansion book (booklet really) that introduces new rules which will allow you to play Descent as a rules light roleplaying game.
I should probably also include a word of caution here, this is just a little 11 page book which includes the basic rules you need to play Descent as a roleplaying game. It may not be what you would write if you wrote a Descent RPG, but you didn't write a Descent RPG so check this one out instead!
It's meant to play much like a session of D&D 4th Edition (assuming your games go like mine), where you roleplay your characters, do a few skill checks, and then you move onto that sweet, sweet tabletop battle action using the well-defined rules already found in Descent. It reads quickly and concisely, there are really only three pages of game rules, then a guide to creating your own characters, a bestiary to help GMs create NPCs, and finally an overview of some famous places in the Runebound world. And yet it's all you need to get started.
Hope you enjoy it!