After giant war
Nico somehow doubted that there is, was or will ever be a worse hour in his entire life than this hour. That is mainly because he has to sit with Octavian to account for all the deaths that happened. And Octavian . . So, to try to pass time, and ignore the annoying blonde. Nico decided to make a paper plane. By the time the half hour mark came around, Nico was ready to murder the blonde.
He had made close to fifty paper planes by that time. All of them made by him pouring his anger into their making. So, when Jason opened the door to check on them( mainly to make sure that Nico haven't teleported the place to the underworld yet) some wind came through.
Jason entered the room, leaving the door open. He saw all the plains on Nico's table. At this point Nico was asleep as Octavian continued talking and writing on the board in the room. Jason decided to throw one of the planes in the air to see how long it would stay flying.
The plane flew alright. Right into Octavian's back. When he turned around, he found Nico and Jason gone, and forty nine airplanes flying around the room. He tried to avoid them to the best of his ability( which wasn't much) but then he got hit with one in the face.
Octavian died because Nico was the one who had all the planes in the air. He died while Jason took Nico to the closest McDonalds to cool off.