jerk-ass ex machina
A god-like, magical, or other powerful force that is around the absolute least amount required to keep a plot moving, but is not exploited or around enough to be more useful than minimally necessary.
"The eagles in 'The Lord of the Rings' and the sentient ocean in 'Moana' are jerk-ass ex machina: They could have solved the entire problem right away by taking the McGuffin straight to its destination without subjecting anyone to a perilous adventure filled with murderous obstacles."