Dolofónos, God of Assassins.

Percy and Cole have been betrayed by those they loved, other than a few close friends. After saving Olympus repeatedly, they are shunned and feared for being too powerful. In the midst of their dilemma, Lord Chaos comes to give them another chance at life - Percy as Commander Alpha, and Cole as his personal assassin Dolofónos, as well as Admiral Omega of his army. Years pass, and while peace is brought to the universe, trouble brews on a familiar planet. How will past allies react to the truth of the past, and will Percy and Cole be able to face their pasts?

Percy Jackson/ Heroes of Olympus fanfic