Straight: A benevolent and powerful spirit is sealed away in a mystic mask.
Exaggerated: A legion of 777 noble and powerful angels, any one of which could resolve the conflict alone, are all sealed away inside one outrageously complicated prison...All of them.
Downplayed: The spirit was just hanging around in a difficult-to-access cave. He's a good guy but far from being a powerful savior.
The Big Bad was either not powerful enough to kill her, or otherwise doesn't have the power to do so, and so decided to do the next best thing: Seal it away and try to make sure that no one ever finds him/her.
No longer needed but realizing that the Big Bad would eventually return, the spirit seals him/herself away so he/she will still be around when the bad guy does return.
The Big Bad makes sure that no one can free the Sealed Good in a Can.
The spirit has taken precautions to become impossible to seal away by any means.
Discussed: " You'd think they'd cram me inside a bigger can, but no! I have a killing 800-year neck strain right now!"
Conversed: "This is the part when they release the Spirit of L Ife from the mask."
Implied: The Big Bad mentions a powerful rival and how he/she is no longer a problem to him where he/she is now. We never see said enitity nor his prison.
Deconstructed: Anything evil enough to seal away this force of good would make sure it's not just a sort of peaceful eternal sleep. The force of good has been living in a nightmare world that would make Cthulhu wet himself in terror for the last thousand years, and has gone completely insane as a result. When the heroes do let it out, it eats them and goes on a murderous rampage before realizing what it's done - and kills itself.
Anything evil enough to seal away this force of good would make sure it's not just a sort of peaceful eternal sleep. The force of good has been living in a nightmare world that would make Cthulhu wet himself in terror for the last thousand years, and has been severely traumatized as a result. When the heroes let it out, it babbles incoherently and hugs them before breaking down in tears. With the help of the heroes, the entity slowly regains its sense of identity and purpose and helps in the fight against the bad guys.