How to deal with two "chosen ones"?

[ThinkerX]

In my universe, demons are in fact fallen angels. The thing is, however, that there were so many angels who fell that they started to burst out of Hell and terrorize humanity. In response, the mother goddess sealed them away in "a place beyond mind" and gave the 'key' to this place to her acolytes to keep safe. The key could only be accessed by a dangerous and taxing ritual that was passed down through generations. Eventually, it reached an unworthy acolyte who allowed the demons to escape after they tricked him. This time, the angels had to intervene and the mother goddess' acolytes were shunned entirely, quickly becoming an isolated cult. The mother goddess fell asleep unexpectedly and the angels, not knowing what to do, destroyed a large portion of demons and sealed the rest of them away (again). The result of their indecision was that the demons had time to populate the earth with demon-human monsters. Fast forward about 1,000 years and about 40% of the population of the planet has demonic ancestry (essentially making them spooky elves rather than monsters), D is the new heir to the 'key' to the place beyond mind, and a nearly angelic princess (C) is born right as D leaves the cult.

Suggestion:

Situation like this - the 'great evil locked away behind a crumbling barrier' - most authors go the 'static route.' The demons were evil then, so they remain evil after hundreds or thousands of years, and once unleashed will be an unstoppable destructive force. That's overdone and limiting.

Were it me, I'd go a different route: these demons showed the ability to change their goals once before - the original rebellion. Its been a long, long while since they were locked up - time enough to change those goals again. So, door opens, cultists at the ready, others expressing apocalypse, the demons go, 'nah, not interested, we got other things going on.' Or they discovered how to reach some other world or dimension. Or, if you wanted grim and out of left field, then some other entity or group there of gained access to the prison dimension and wiped most or all of them out.