Maggot has been alone for as long as he can remember, all he can remember of his parents is their disease ridden corpses in a quarantined house. He was forced to do whatever he could to survive, from begging, to odd jobs, to stealing when he had to, leaving him constantly on the move. Most recently he's taken to the unfortunate career of graverobbing, taking what valuables he can from the deceased to survive. The most valuable rewards being that from deceased cultivators, but those are hard to find as most who die in battle have their valuable taken by the victors, or they die in some area that a normal person would die upon reaching.
But the war/massacre of the Red Dew Baron's forces was an incredible boon, as a battleground filled with dead cultivators would likely have many valuables he could sell. The battleground in question was well picked over, but he found a broken sword hilt that was covered in fine engravings that looked like it could be valuable. However, as he touched the blade he cut himself, leaving a drop of blood on the hilt which awoke a binding in it.
Suddenly a terrifying warrior figure appeared and condemned him for stealing from him. The Soldier then demanded recompense for the boy, who they named Maggot Gravedirt for their lowly stature, or they would end their life there. The boy now known as Maggot promised to do anything, so the spirit used the blood spilt to force a 'symbiotic' connection with him.
This ignited and warped Maggot's core, awakening him as a cultivator, but permanently binding him to The Soldier. The Soldier wanted revenge against the one who'd killed him, but since The Red Dew Baron was deceased, it wanted revenge on the Overlord who ordered the Baron to go to war. To do that it need Maggot to be a stronger vessel, so forced him to march without sleep to the nearest sect - which just so happened to be The Red Dew sect.
Notes: I see this character as a mix of the xianxia trope of an old spirit guiding a young cultivator, combined with the anime Shaman King. The Soldier can possess Maggot to make him fight better, but also wants him to get stronger independently so it doesn't break the body. I don't know what plans you might have had for the force the Red Baron battled, but I tried to keep it vague so it won't interfere with your plans there. I planned that the Elders wouldn't know immediately that this boy is sort of possessed, but I also feel the Red Dew sect might not be that angry if someone wanted to become strong enough to get revenge against the Overlord who ruined their sect.
But let me know if this doesn't work or what I need to change.