Will and Nico were on a date and from moving them around Nico overshadow travelled himself making him tired, causing Will to take Nico back to cabin thirteen (Hades cabin) and lay him down on the bed while Nico passed out from exhaustion Will went to remove Nico’s sword from his hip.

When Will went to remove the sword he accidentally cut himself with it and the sword being Stygian iron it would usually absorb the souls of people so when Will passed out Nico woke up and immediately looked panicked realizing what happened.