The following article contains spoilers from Future State: Gotham #13, on sale now.
Ever since his brane overdose, Nightwing has been acting in opposition to some unknown threat to the world at large. To that end he has sent Talia al Ghul to free Bruce Wayne, so they could learn the fate of their son, and even reclaimed the title of Batman for himself. All of this was in preparation for a threat he has elected not to inform the others about. Future State: Gotham #13 (by Dennis Culver, Dan Mora, Geoffo, and Troy Peteri) however, seems to suggest that Nightwing's plan to save the future, might be what puts it at risk.
The whole reason Nightwing overdosed on brane was to use the powers it would grant him to find Damian. After his visions, however, his plan to save the world seemed to somehow involve rescuing Damian. Yet, the issue ended with Damian returning as Batman 666, seemingly hellbent on getting revenge against those who wrong him. This could mean that in trying to save his brother, Nightwing allowed the world to be put in danger.
For those who have followed Damian's story, there have been several possible futures presented for him. The worst-case scenario is the one where he becomes Batman 666, a lethal vigilante who made a literal deal with the Devil for immortality. In that dystopian future, Damian exists in a post-apocalyptic world and is heavily implied to have had a hand in creating it. So, for him to reappear in the future state as Batman 666 is troubling, and lends to the theory that he is the threat Nightwing foresaw.
Yet, Nightwing also had to have known what Damian's ultimate fate is. His visions have been so accurate that he couldn't possibly have been ignorant of how dangerous Damian has become. He had to know that the quest to find his lost brother would inadvertently free him from hell so that he may seek his revenge. By telling Talia what she needed to know and where she needed to go, Nightwing has knowingly unleashed a potentially world-ending threat.
So, why take that risk? Surely if he is acting as an emotionally dead, yet practical hero then he must have suspected leaving Damian there would be safer for everyone. That isn't how Nightwing operates though, he leads with his heart, not his brain. His visions also seem to have shown him the path to not only averting catastrophe but reuniting his family. It all starts with getting Damian back.
The two have always been close. Damian was Nightwing's Robin for a time, and he's the one Damian looks up to the most out of all his siblings. So, it's no surprise that Nightwing would be willing to risk anything, even the end of the world, to save his little brother. If so, then Nightwing's hand in enabling the start of the apocalypse is actually a huge leap of faith and one that proves how much he loves and trusts Damian. He knows the path ahead won't be easy, but everything he is doing, risky as it is, is to save his family.