This is an official post by the Antibirth team. Proof here.

Five years ago, a young boy took his first trip down into the basement, facing the endless threats below to escape from the murderous delusions of his loving mother. Three years later, he found himself facing the same predicament but with an even greater plague of horrors and peril. Very shortly afterward, a modest few among us decided that even this was not enough. In the two years since then, fate has worked tirelessly to place by far the most fearsome challenge yet in his path. This time, Isaac will face our free and fan-made The Binding of Isaac: Rebirth expansion, Antibirth. So what exactly is in store for him?

• 2 new characters with unique playstyles
  
• 90+ new collectible items to help Isaac fight
  
• 18+ new trinkets to swap and strategize with
  
• 80+ new enemies to face one after another
  
• 16+ new bosses to kill or be killed by
  
• 4000+ new rooms to explore and survive
  
• A new secret final chapter for Isaac to endure
  
• Bigger and greater secrets than ever before
  
• Several new or enhanced features for local cooperative play
  
• A nearly full and original new soundtrack, evocative of classic Isaac
  

The Binding of Isaac has had one of the most involved and robust communities of fans around since its initial release and has inspired countless players and developers with its peculiar sensibilities and creative blend of roguelike flavor. As its zealous modding community has proved, it's only natural that many among us would feel the urge to try to help carry the torch and make it partly our own, but Antibirth was the first and is presently the only Isaac mod to reach deep into the core of Rebirth to enable the depth and scope of a full expansion, aspiring even to the size of the official DLC, Afterbirth. If you've become as enraptured with Isaac as the rest of us and you still want more, we're here to provide our absolute best, and more of it than you've ever dreamed of, all for free.

With the nature of this mod's downloads, some antiviruses may act up when attempting to download Antibirth. The downloads from antibirth.com are completely safe to use - you should be able to download this mod without any issues.