Throughout its three seasons, "Trollhunters" features several major plot twists. A key early reveal is that Jim's history teacher, Mr. Strickler, is a Changeling, a troll who can take human form, and is working for the Gumm-Gumm army. This complicates Jim's life immensely, as Strickler is also dating his mother. Another significant twist is the death of AAARRRGGHH!!! at the end of Part 1, who sacrifices himself to save Toby, though he is later revived in the Darklands. In Part 2, the ancient troll leader Vendel is murdered by Usurna, who reveals herself as a traitor working for Gunmar. This act dismantles the leadership of Trollmarket and forces the heroes to be more self-reliant.
The most significant spoiler of the series is Jim's transformation at the end of Part 3. To defeat the seemingly invincible Gunmar, Jim willingly undergoes a magical ritual devised by Merlin that permanently transforms him into a half-troll, half-human hybrid. This act is the ultimate sacrifice, as he gives up his full humanity to gain the strength needed to protect everyone. He succeeds in killing Gunmar, but this victory is immediately overshadowed by the return of the ancient sorceress Morgana, who possesses Claire. The final battle sees the team defeat Morgana, but Arcadia is left ravaged, and the trolls must leave to find a new Heartstone, with a now-troll Jim leading them.
The entire saga concludes in the film \"Trollhunters: Rise of the Titans,\" which contains the ultimate twist. After defeating the Arcane Order but at the cost of Toby's life, a grief-stricken Jim uses the Time Stone to travel back to the first episode. He chooses not to pick up the amulet himself, instead guiding events so that Toby finds it and becomes the new Trollhunter. This ending reframes the entire series as one possible timeline, suggesting that the core lesson was not just Jim's journey, but the knowledge he gained to ensure a future where his friends could survive and thrive, even if it meant giving up his own heroic destiny. This reveals that the true \"Trollhunter\" is a mantle of leadership and mentorship, not just of power.