A drug dependent father finds himself part of an epic battle against a demonic legion and is joined in the fight by a time traveler from the future as he learns the fate of the world lies within the mind of his autistic child.

Sounds like an epic battle indeed, but there seem to be too many elements and as such I can’t decide which is the most important. How about missing out the bit about a time traveller, interesting as it is, to focus on the father and son? “A drug dependant father protects his autistic son against a demonic legion as he discovers the fate of the world lies within the boy’s mind”.