Project Eternity's aim is to bring back the style of the old Infinity Engine games. Games such as Baldur's Gate and Icewind Dale. However, there's one aspect of those games that Eternity will be very different in. It won't be based on the Dungeons and Dragons Rule set.
So Obsidian are creating one from scratch, which Chris Avellone explained to us at Rezzed, “The two people heading that up in the studio are Josh Sawyer, the Project Director and Tim Cain, one of our gameplay programmers. Tim was Arcanum, Fallout, Temple of Elemental Evil , and Josh was New Vegas and Icewind Dale, among others. They both really like designing systems and that's been proven throughout both their careers, so the idea of creating an RPG system like that from scratch is an idea they're really excited about, it's been pretty amazing to watch.”