10. Onaga's Reveal
Throughout the Konquest Mode of Mortal Kombat: Deception, you play as a young warrior called Shujinko. One day, an ethereal spirit called Damashi encounters Shujinko and claims he is the only one who can stop the resurrection of Onaga the Dragon King.
Since Onaga's return could end all existence, Damashi agrees to aid Shujinko on his quest. Our hero travels throughout multiple dimensions to locate the only artefacts that can defeat Onaga; The Kamidogu.
Over 60 years later, Shujinko manages to track down each of these items and places them in the Nexus of reality to destroy Onaga's essence once and for all. Only then does our hero learn that Damashi IS Onaga! ("Damashi" literally means "Deception." That should have been his first clue.)
This means that Shujinko spent his entire adult life helping this malevolent entity, under the false pretence he was preventing his return.
What's more shocking is that Raiden's ending shows the thunder god slaying Shujinko for his blunder. Even though Shujinko was pure of heart, Raiden felt obligated to kill him since his folly nearly destroyed the entire multiverse.