Even though Batman is one of the darkest heroes in DC, he's also one of the greatest, but that darkness comes with a few unhealthy habits, none of which is more unhealthy than his birthday ritual.
When Batman lost his parents in crime alley to the mugger Joe Chill, it was the darkest moment in his life, and it set the entire course for the rest of his life. From that moment on Bruce began training until he could eventually become The Dark Knight that could protect Gotham and all the innocents in it so that they'd never suffer as he did. To this end, Batman has created a training program that he uses every single year on his birthday, and it is such a dark ritual that even Alfred and Damian have expressed concern over it.
In Detective Comics #999 by Peter Tomasi and Doug Mahnke, Batman is pushed to his limits, after watching everyone he knows or loves die, Alfred, Leslie, Ducart, Kirigi, being chased by a horrible amalgamation monster of his worst villains, Bruce finally finds the source of all of this: it's a younger version of himself. The mastermind behind everything Bruce suffered that day was himself because every year Bruce designs a virtual training environment, designed to push him to his absolute physical and mental limits. Because Bruce needs tragedy to fuel him, he needs to be able to continue on his quest even if everyone he loves dies. Training himself to fight against the absolute worst of his villains, even if they all merge together into one, is how he accomplishes this. The revelation that the horrors Bruce just faced were entirely designed by himself is exceptionally dark, but before he can leave the simulation there's one more dark ritual he needs to complete.
Batman Forces Himself To Relive His Trauma Every Year
After confronting the child version of himself that was running this simulation, Batman has to do one final thing before he can leave, which is to bury his childhood self in a grave alongside his parents. As the child version says, "Three people were murdered outside the Monarch Theater that night." The price of being Batman, the price of saving all the people that Batman saves every single year, that price was Bruce Wayne, his childhood, and his innocence. And every year Bruce has to relive the worst moment of his life, by burying what that night cost him, just so he can continue being The Dark Knight that watches over Gotham. This dark sacrifice is shown perfectly in Bruce's final exchange with his child self. "I still wish you could've had a chance to be a kid just a little longer," "It's okay. So many other children have gotten to because we didn't."
Batman is one of the greatest heroes in the DC universe, he fights against some of the darkest villains DC has to offer, true monsters like Scarecrow and Joker. But in order to continue being The Dark Knight, Batman has to remind himself of the cost he paid to become Batman, and every year, that cost continues to be worth it.