I had a campaign where the climactic key moment to "win" involved the party fighting their way up to a portal to hell and then casting a gate spell from a scroll to permanently close it. Gate is 9th level, though, which was above the level the party wizard was capable of reliably casting. Rather than let the resolution of the campaign hinge on random chance, I provided the party with an NPC who was capable of casting 9th level spells, but who had various reasons for being incapable of simply blowing through enemies for the party. I thought of him as my "Gandalf" character.

In the final battle the party wizard managed to reach the portal, and walled himself in with a resilient sphere so that he'd be able to use the scroll in his next turn without being disturbed. Fortunately my Gandalf still had a teleport spell available to him and teleported into the sphere next to the party wizard, offering to take the scroll and use it to save the world.

On the party wizard's next turn he used telekinetic shove as a bonus action to shove my Gandalf through the portal to hell, and then once the Gandalf was through he tried casting gate to permanently seal the world off from hell. Turns out he'd never trusted my Gandalf, suspecting him all along of being secretly evil, and so he backstabbed him at that crucial moment before my Gandalf could backstab him. There was literally nothing in my Gandalf's repertoire of remaining abilities that would have allowed him to overcome this, so off to hell he went.

And then the party wizard flubbed his attempt to cast gate, using up the scroll in the process. :)

Fortunately I'd prepared a backup cinematic for that outcome, the flubbed casting was still able to collapse the portal to hell and ruin the bad guys' plans. It just left the world in a much more "interesting" situation than the party had been aiming for. I frankly preferred that outcome anyway, so I'm not even mad. Though my Gandalf was rather peeved.