Treacherous Advisor/Playing With

Basic Trope: An apparent ally of the heroes who offers advice and support... up until The Reveal that they were a villain all along.
 * Straight: Professor Percy has been helping the heroes investigate and collect the Prismatic Pearls of Power. However, after they find the last one, he reveals that he was using them all along.
 * Exaggerated: Not only was Percy a bad guy all along, but he's actually Lord Evildoom, someone the heroes had heard about but never met -- or, rather, never realized that they'd met.
 * Justified: Percy knew where the pearls were and needed someone to go get them. Why hire a mercenary group when you can con people into retrieving them for free?
 * Inverted: Sarcastic Devotee
 * Stealth Mentor
 * Subverted: It looked like Percy was going to betray the heroes and give the pearls to his master, Lord Evildoom, but he's actually a double agent working for the common good and switches the pearls for fakes that ruin Lord Evildoom's immortality ritual.
 * Double Subverted: ...but then Percy betrays the heroes as well as his former lord, in order to acquire immortality for himself.
 * Parodied: Percy has an Evil Laugh, an Evil Shadow, Evil Hair and Evil Clothes. The heroes STILL don't see it coming.
 * Deconstructed: Because they didn't suspect Percy, the villain’s plan succeeds, which results in the deaths of several civilians. The heroes, full of guilt that they weren't able to prevent it, have a hard time trusting anyone ever again. During the next crisis, they constantly accuse their advisors and each other of being double agents, and their infighting lets the villain win again.*
 * Reconstructed: To make sure that sort of betrayal never happens again, the heroes get better security methods such as background checks and reporting any suspicious behavior. Though Percy made them all suspicious of each other, they realize they'll have to trust each other to get anything accomplished.
 * Zig Zagged: It looked like Percy was going to betray the heroes, but he's actually a double agent, but then he betrays the heroes as well, but it turns out that this was to avert the resurrection of the demon guardian of the pearls, but it turns out that he was actually doing it to cause the demon's resurrection...
 * Averted: Percy does not betray the heroes.
 * Enforced: ???
 * Lampshaded: ???
 * Invoked: ???
 * Defied: ???
 * Discussed: ???
 * Conversed: ???
 * Played For Laughs: Professor Percy is rather obviously Lord Evildoom in a Paper Thin Disguise, but Bob and the others were still clueless... save for Leon, but nobody ever listens to him.
 * Played For Drama: Upon The Reveal, Lord Evildoom uses the Prismatic Pearls of Power to attack the heroes and capture Princess Elisa.

Once you've collected the Mac Guffins, head back to Treacherous Advisor...sucker.