In Season 5, Episode 13 ("Labor of Love"), Hercules is trapped in the Underworld and, like all who are down there, they must complete their unfinished business before moving on to "a better place". However, Megara is also down there and is able to leave to a better place at the same time as Hercules.
But what was her unfinished business? To be a damsel in distress and get saved by Hercules?
Megara's unfinished business was to help Hercules complete his Twelfth Trial. Her death occurs while
running away from Cerberus at the same time Hercules is felled by the beast.
Hades (both in classic mythology and OuaT) has a singular purpose: prevent souls from leaving the Underworld. To this end, concerning Hercules and Megara, Hades
locks up Megara as a prisoner for 40-ish years under guard from Cerberus. This prevents Hercules from finding Megara, which would give him the confidence and/or drive to challenge and defeat Cerberus (and in the OuaT mythology, and actual helping hand in the battle).
This effectively prevents both Megara and Hercules from finishing their unfinished business, notching two stuck souls for Hades. After completion of the Twelfth Trial, both Hercules and Megara have completed their unfinished business (the actual completion and helping with completion, respectively), and they are free to leave the Underworld for "a better place".
I was left with the impression that her unfinished business was leaving Hercules to fight Cerberus on his own and that by helping him in the fight in the underworld she was able to resolve it.
However I also think there is probably more to the story that involves her intentionally setting Hercules up to fall which would make her helping him fight Cerberus make much more sense. But we would need newer episodes to confirm that for us.
