"Will they manage to succeed this mission and bring harmony/balance to their rightful place?"- well for starters, every demon would rather have hell destroyed than spend one more second there. Harmony/balance? No demon would ever seek that. Maybe you could make the demons embark on this mission with the ulterior motive of killing the angels on their group/ensuring that destruction DOES* happen. So even if they say they will pair up with the angels to save the harmony, their true intention is to STOP* them from doing so.