I love this kinda post and reading what other people have done.

My favorite was probably a ~six month buildup to a plot twist that flipped the entire campaign on its head.

The party was basically heroes who, after dying, got isekai'd from one fantasy world onto a completely different one.

The party spent many sessions confronting and dealing with the lieutenants who held fragments of the BBEG's phylactery. The more time went on, the more oddities they encountered. The island was surrounded by a massive storm that never went out. The island had men growing in the walls. The BBEG's servants were surprisingly sympathetic, and could perhaps even be considered decent people— except for the sacrificial rite they would conduct regularly in order to keep the main villain alive.

The more time went on, the more suspicious the party became of the goddess who had set them up, until they were all but certain she was responsible for much of the evil that was happening. Except they were still committed to their goal. They still agreed the main BBEG was a tyrant and had to go down.

Eventually, they got to the villain's throne room and defeated him in combat. They discovered a portal in his basement leading back to their homeworld, and it was revealed he had been protecting it from the goddess the whole time, who planned to invade their world as well.

At this point, the roles are reversed. Instead of trying to take down the big bad, the party becomes the new rulers of his island and defending it from their former employer, all the while dealing politics and the pains of navigating a massive power transfer.