The final third of Dragon Quest III is just one looooong holy shit moment right up to the game's ending, and was absolutely the inspiration behind one of the more infamous gaming moments (Kanto in Pokemon G/S/C) posted on the first page. Specifically:
In the first Dragon Quest game, you play as the descendant of a hero who saved the world from darkness. To do so, you traverse a mostly open world, gathering items and weapons that your ancestor was said to have used to reach the game's main villain and defeat them. Dragon Quest III seems relatively unconnected to the previous games at first, as the scenario is more about avenging your father and defeating a powerful demon lord. After said demon lord is defeated, everything seems all well and good... until it's revealed that said demon lord was only a flunky for the game's true villain, who resides in another world. The party gives chase and ends up in this world of darkness that the antagonist was said to lord over. You land in a pretty unassuming pier, but when you step out onto the world map... some familiar overworld music starts playing and it becomes clear that the world of darkness is in fact, Alefgard, the world of DQ1.
So the last act of the game has you acquiring items that you'll need to fight the main antagonist, mirroring the journey that the player character takes in DQ1, with towns that were depicted as destroyed in the first game being populated and full of life, and characters that were only mentioned in the past tense in DQ1 being alive and well here. The game's ending confirms what most players have probably pieced together by that point: Dragon Quest III isn't a sequel, it's a prequel to the original Dragon Quest, and you're playing as the legendary hero spoken of in that game.
I only recently played through DQIII, but somehow never had that twist spoiled for me, and it's stuck with me ever since. Absolutely masterful.