Shortly after the announcement of the release date for Binding of Issac Rebirth's next big DLC, Afterbirth; the BoI fan community have announced a big fan-made DLC pack of their own.
The BoI community are known for the thriving modding that a good deal of people within it get up to, so it's probably not such a big surprise that this is happening. Titled Antibirth, it's almost equal in size to Afterbirth, with two new characters, over 16 new boss monsters and over 90+ new items for Issac and the other characters to find, equip and warp themselves with. The biggest thing of all that it adds though is a 'true' co-op mode, in which both players will control a character instead of the main player being a character and the secondary player just a floating baby familiar of some odd appearance. There's also an original soundtrack and a secret new chapter for those who dare to seek it out. So, it's fairly impressive an effort.
As a mod for the base game though, Antibirth and the official Afterbirth DLC cannot run at the same time, although it is possible to use the same base game to run each pack at separate times - Afterbirth saves will not be affected by Antibirth runs either. The team behind Antibirth are planning to get their pack supporting all the DLC eventually though, including the upcoming Afterbirth. Heck, given that Afterbirth is introducing a function where the best fan-made mods get added to the game permanently they might even get help from the game's makers themselves on that front.
For now Antibirth is only available on Windows machines. The team behind it are planning to improve upon and update the pack as time goes by, so hopefully Mac and Linux users will be able to play it without having to use a virtual machine in the future.
Antibirth will be free to download when it launches on December 20th. There's even a trailer for it as well as an official site, so keep your eye on this one. It promises to be quite amazing.