The Binding of Isaac: Eternal Edition (also dubbed Eternal Update or V1.666) is an official free DLC-like expansion to The Binding of Isaac, which adds a new Hard Mode, as well as fixing some long existing bugs.
The DLC was developed by Florian Himsl (the original programmer of the game) and released for free on Steam on May 05, 2015, for anyone who owns Wrath of the Lamb DLC.