WARNING: The following contains spoilers for Hellions #9 by Zeb Wells, Stephen Segovia, David Curiel & VC's Ariana Maher, on sale now
Mister Sinister is one of the most dangerous members of the Quiet Council. A master manipulator and devilish genius, Sinister has quietly been playing his own game in the background of Krakoa, filling up his genetic banks of mutant DNA. He's even been quietly recruiting mutants into his service such as Psylocke, holding the fate of her daughter over her head in exchange for her compliance.
But it appears Sinister has become a little too overconfident since he just allowed a minor mutant to gain the upper-hand on him in Hellions #9. During a supposedly friendly chat with the Silver Age villain Mastermind, Mister Sinister is caught off-guard and betrayed, and it could set up serious problems not only for the Hellions but the X-Men as a whole.
Mastermind appears to be one of the mutants approached by Mister Sinister for a potential arrangement. Mastermind still works with the Hellfire Club, albeit in their new form as the Hellfire Trading Corporation, but seems to have found some unlikely empathy with Sinister. Mastermind suggests that they both "bristle" against the lines drawn and the laws enforced by Krakoa. After some discussion, Mastermind seems to agree with Sinister's plans and offers to work with him. But the tea Mastermind presented as a courtesy was all a ruse and quickly proves to be a powerful enough poison to even affect Sinister. Mastermind only has to sit back and watch as Sinister falls out of his chair screaming in pain. The villain can only crawl a few feet away before collapsing into unconsciousness.
Sinister's inability to see the betrayal coming means that the Hellions need to find their wayward leader, not just for their own sake but for all of Krakoa. As a member of the Quiet Council, Sinister's abduction is a terrible precedent that the island can't ignore. The Hellions are assembled to hunt him down, but this only invites more trouble, as Mastermind uses the opportunity to trick Sage and capture the Hellions as soon as they reach New York,, handing them over to his benefactor, Arcade.
It comes across as a terrible mistake by Sinister and one that could potentially endanger a lot of lives. It's not like Mastermind has a sterling history of integrity, either. His role in stories like The Dark Phoenix Saga all centered around subterfuge and manipulation. Since being restored to life, his only notable act to help Krakoa was taking part in the battle against the Cotati aliens during Empyre.
Mastermind didn't even need his illusions to defeat Sinister, instead beating the long-time X-Men adversary with a cup of poisoned tea. Sinister's musky cape even inadvertently helps to hide any smell of the poison. Unless this is all a feint to find Arcade -- which seems unlikely given how Sinister is shown captured shortly after -- then the master manipulator just set himself up for a major defeat.
Sinister's mistake in letting his guard down around Mastermind for even a moment may now have gotten his entire team into hot water and set up Arcade to start a new campaign of terror against the X-Men.