How I Joined the Hero's Party Despite Being One of the Ten Elites

I am the Ghost Prince, one of the Ten Heroes of the Demon King's castle. When I returned to my homeland to visit an old friend, I witnessed the hero's party tormenting my companion, the Sand Dragon. I was so focused on avenging the Sand Dragon that I forgot I could only use a tenth of my strength under the sun. That bastard hero defeated me, and when I woke up, my body had become physical! The Demon King told me that if the hero's party were left to act on their own, no one knew what would happen to the balance of the game. So, at his command, I concealed my identity. When the hero asked my name, I panicked and gave him the alias Kaige, an adventurer. For the sake of the Demon King's grand cause, I joined the hero's party on their quest to save the princess. My powers may be sealed, but the hero is just as easy to fool. The all-capable Ten Heroes will claim the princess for the Demon King's castle! *1. A crossover of otherworldly settings, mainly Sleeping in the Demon King's Castle. I may also include The Hero Is Overpowered but Overly Cautious, That Time I Got Reincarnated as a Slime, and others. 2. Pairing undecided. The protagonist used to be human. P.S. I don't know why, but I just think this blurb really suits this kind of trope, hahaha. Theme: Overcome every obstacle and build a new world of peace and beauty.