There was a reason why Nico di Angelo stayed away from Camp Half-Blood, and it was a reason no one knew.
He kept it secret for four years. For four years he lived in the Underworld, in Camp Jupiter, anywhere he could find to stay away from Percy.
And then on the quest for Diocletian's scepter, he found Cupid – or, more accurately, Cupid found him – and Jason found out.
Nico's terror was unimaginable. He knew things had changed since the thirties, but he was still afraid. And even though, in his heart, he knew that Jason would never hurt him… He still felt the pain that came from four years as an exile, an outcast, a child of Death and the fear that came along with all pain. And with the fear, came the darkness, stronger than before and threatening to break out if Nico didn't contain it, threatened to destroy and kill.
And so he closed himself in. He built walls in his heart and tried to concentrate on the mission, the mission to save Percy.
And for a while, for a few days… That gave him focus.
And then Percy came back from Tartarus and the walls fell down.
So he took the Athena Parthenos to Camp, staying away from Percy and the rest of the Seven, staying away from the people he loved most and cared for the most, so he wouldn't poison them with his darkness which he felt inside, clawing to get out, and he let it go, just once, just once he unleashed his pent-up anger and fear…
And killed Bryce Lawrence. He sent him to the Underworld, to Punishment. He had never killed like that before. And he remembered why he stayed away, and the darkness, and he knew more than ever that he was not welcomed at Camp Half-Blood. And he pulled back. He could have killed them all, killed Reyna and Coach and the Romans and the monsters and Annabeth and everyone else, but he remembered who he was and he held back.
Then he brought the Athena Parthenos back to Camp, and went to assassinate Octavian. He was ready, he was ready to go to the command tent, suck Octavian's agonized life force into his Stygian blade, kill him the same way he'd killed Bryce Lawrence.
The darkness was inside him. The darkness wanted to be released.
And then Will came.
And the darkness left Nico like a shadow at sunlight.
And Nico realized that he could never leave Camp Half-Blood, not while Will was there.
Will Solace was killed by a monster in the forest. Nico felt the darkness inside him again, darkness stronger than before for it's neglect, darkness that wanted an outlet, an excuse, a reason to destroy and hurt and kill.
And when he found Will's body in the forest, the darkness came upon him, and in his moment of weakness, for the first time in his life, even more than at Westover, more than Tartarus, more than when Percy fell into the pit and when Bryce attacked, more than at his most hopeless moment – Nico gave in.
And the darkness had free reign.
And this time, Nico didn't stop it.