After you got to camp and got claimed by Hades, you were going to need someone to teach you how to use your powers. Chiron decided that was going to be Nico, since you both share the same/similar powers.

You and Nico were in the training arena at night, as he's trying to teach you how to shadow travel, getting annoyed at the same time though.

"You have to focus on where you are going and it's really tiring so have to stay awake or else you'll end up in the underworld or something. Now try, to go through the shadows, and go to the other side of the arena." He instructed.