I'm strung out for clever names, ideas? Anyhoo. When a wizard, scientist, (or both!) create a can to seal evil into, a suit of armor that can withstand all harm, and a weapon capable of felling anything it strikes in case the evil should somehow get loose (hey, at least they're planning for the inevitable), they must all have a key or weakness that can activate, deactivate, or destroy it. It's unstated, (at least I haven't found a work that explicitly talks about this) but magic, technology, or whatever, can't make a truly inescapable prison, unbreakable armor, weapon that can One-Hit Kill, or unsolvable puzzle. These artifacts or devices always have a limitation or weakness that somehow enables their power, like a keystone. This is probably just to avoid creating a Mary Sue object, and to give the villains and hero a way to fight each other. However it might be a form of intentional irony, the only way to make these things inescapable, unstoppable (or basically anything you could add the in/un prefix to) is to make it stoppable by at least one thing. It can be a key, a weak point, not being able to harm virgins, a remote control or something more exotic. If the weakness is a Power Source Soul Jar, or a form of remote control, destroying the "Keystone" destroys the device/weapon/person/army.
Hello, Unknown Troper. You'll need to get known to lend a hand here.