Bloated blood-burping bumbleflies, electrotears, and so many more secrets are now buzzing around the basement in The Binding of Isaac: Rebirth, thanks to the release of a huge and hugely impressive community-made expansion. Yes, the second official expansion launches in a fortnight but before then absolutely do check out the free Antibirth [official site] mod. Antibirth is mahoosive, adding new characters, new enemies, new bosses, new items, new music, new... it's really big, and really well-made too. Also, this trailer is all sorts of creepy:
So! Antibirth brings over 90 new items, over 18 new trinkets, over 80 new enemies, 16 new bosses, a new soundtrack, new room layouts, a new secret chapter, new odds and ends all over the place, and two new unlockable characters. I've only seen one of them, Bethany, who... turns soul hearts into attack orbs or something? So many mysteries! Antibirth also tweaks a few things to work more like pre-Rebirth Isaac.
I've had a look at Antibirth myself and am mighty impressed. New enemies and bosses have familiar Isaac abilities and characteristics put together in new and interesting ways. The new art is peachy. The items are mostly mysterious to me, which is great, but I do like what I've understood. My favourite so far is 120 Volt, which turns tears into electroballs arcing shocks between enemies (hey, you've got to have an overpowered item or two).
I'd say Antibirth is more difficult than Official Isaac, though I couldn't say to what extent given that a lot of mastering Isaac is simply understanding how things work. I'm jolly keen to play and see and learn more.
You can grab Antibirth from its site. You only need Isaac: Rebirth to play, as it currently doesn't use the Afterbirth expansion. That makes it feel doubly like a weird alternate universe expansion, as it's missing items I'm used to having around.
Antibirth is made by Public Entertainment Networked Interactive Software, who did this all without the official mod tools due to arrive with Aft- heyyy wait a minute! PENIS. Those scamps!