I Imagine Superman Still Has His Powers Despite There Not Being A Sun - Tumblr Posts
So, I've seen people saying Constantine should be the one who knew about it prior, and that's all well and good. But consider: Superman.
After someone learns about Superman's whole 'World of Cardboard' mindset, they decide that he needs a chance to be in a world of stone. Or ectoplasm in this case.
So Superman gets dragged off to War Day. At first, he's a bit hesitant, but then he sees someone get smashed to bits and reform with a wild grin on their face and decides to just go for it. Maybe he even meets some Kryptonians who's spirits became ghosts who encourage him to let loose.
By the end of the day, Superman has never felt more alive, and has a dozen new friends as well. Plus some people who can tell him more about his home planet before it was destroyed.
When the League ends up in the Infinite Realms on War Day, they end up coming across Superman (who had taken the day off) in a headlock with some large ghost. Superman breaks out of the hold by breaking the dude's arms off. The ghost compliments the how clean the break is as he reattaches his arms before going in for another round.
War Day
After Danny was crowned he decided that the truce party had to have a counterpart, after all ghosts became friends mostly through battles, and he assumed that one day letting them fight wouldn't be a bad thing, it might even help stir up some grudges.
This was how High King Phantom, ruler of the infinite realms inaugurated the long-awaited "Day of War" or just "War Day", a moment when the Infinite Realms naturally became chaotic; alliances were allowed but it was not advisable to trust on them.
And of course, you were free not to participate, you just had to put a blue or green band on your arm, or a little green clock in the backyard of your haunt so the ghosts would leave you, your haunt or your territory in general alone.
Danny thought of it as some kind of giant paintball day, only with no paintballs and full of aggressive ghosts with various powers, it was especially exciting since everyone knew there would be no hard feelings after it and they would end up in the king's palace eating sweets as little children.
They usually celebrated it on a day close to any celebration related to death in human world, when their powers were especially powerful and therefore everyone could have more fun.
The problem was that since Danny had human friends (liminals?) who came to play, they didn't really consider it weird when some humans fell into the realms by a natural portal, and since they weren't wearing any blue or green arm bands they were definitely in the game.
For their part, the family of bats along with some League allies found themselves literally standing on a field of war where everyone seemed to be going for the kill, Jason was strangely excited about it, as was Damian.
When Dick asked one of the locals for an explanation, a guy on a motorbike threw him into the air laughing and yelled "LET THE HUNGER GAMES BEGIN!"