Hell Training As A Default Skullhound

Gates called “Doors” opened up all over earth, and hunters called “The Awakened” have to enter them to kill the demon kings within them- Okay. That sounds generic. Lets add some sort of twist to this. After the hunters have completely cleared all of the doors, the demons are forced to retreat back to the Nether (Hell), and ONE of the demon kings who was killed off easily at first, Gunjo, is abandoned by the Demonic Order, and is stripped from his power and is now a weak skeleton. But when that happens, Gunjo gains a message from a system window: [Stand, Gunjo] Now a Skeleton in hell, he has the system to help him level up and gain limitless Hellbound power, take revenge on the Demonic Order, and escape hell and take revenge on Earth for being defeated by them as well. But things aren’t always going as planned, are they?