The origin of the Batman-Superman 'fusion' in World's Finest revealed

gamesradar.com:

Superman ... Batman - the two most popular characters DC has ever produced, fused into one being. We saw a hint of it a few weeks back on a variant cover of June 21's Batman/Superman: World's Finest #4 and speculated a bit on what it could mean, but didn't have any answers. 

Now it's June 21st, and with the release of World's Finest #4, we finally have an answer to our questions.

But you know what comes next...

Still here? Great, let’s get into it. 

Drawn by Dan Mora, written by Mark Waid, colored by Tamra Bonvillain, and lettered by Aditya Bidikar, World’s Finest #4 continues the saga of Devil Nezha, an ancient evil that will play a part in the upcoming five-issue event series Batman vs. Robin, also written by Mark Waid.

The Devil Nezha is a gifted sorcerer, capable of possessing everyone on planet Earth. Unfortunately for Batman and Superman, that includes other superheroes, meaning the pair must face off against one of their most powerful allies, Hal Jordan, AKA Green Lantern. 

As if the might of a Power Ring isn't enough to make Jordan a threat, Nezha has tinted Lantern's abilities with a strange kind of magic, making his constructs extra deadly toward Superman. The only way the pair are going to survive this threat (and, hopefully, keep their friend Hal in one piece) is to remove his ring. But how will they do it?

Of course, it's up to Batman to use logic to take down a fellow hero, and he comes up with a plan. Though Hal's willpower (the source of his ring's energy) is stronger than both Batman's and Superman's, the World’s Greatest Detective reasons that it is not stronger than their wills combined. So, once the duo are in close enough proximity to Hal's ring to reach it via their own will, they concentrate