WARNING: The following contains spoilers for Hellions #14 on sale now from Marvel Comics.
The mutants of Arakko came to Earth in the fallout of X of Swords and greatly increased the number of powerful mutants in the world. But even those fierce mutants have fears of their own it turns out, and the X-Men should be aware (and wary) of them.
Hellions #14 by Zeb Wells, Roge Antonio, Rain Beredo and VC's Ariana Maher just teased the full danger of Tarn the Uncaring -- and suggests that even the hardened mutants of Arakko are concerned about him going forward.
Tarn the Uncaring was introduced in Hellions #6 by Zeb Wells and Carmen Carnero, during the events of X of Swords. While the majority of Krakoa was concerned with the mutant tournament occurring between their champions and the Swordbearers of Arakko, Mister Sinister was able to arrange it so he and his Hellions could venture into the demonic realm of Amenth. Nominally sent to steal the Arakko swords and force a disqualification, Sinister secretly wanted to harvest new DNA samples for his experiments. Along the way, they came up against Tarn and his creations -- the Locus Vile, a group of vicious cannibalistic mutates.
Capable of molding and manipulating the genes of his fellow mutants, Tarn has also shown a number of other abilities -- such as control over magic that effectively grants him telepathy, teleportation, and telekinesis -- that collectively make him a major threat. Sinister enraged him with his visit, resulting in the deaths of three Hellions, the capture of the Sinister copy and the prompt escape of the other Hellions with the DNA in-tact. Tarn has been biding his time ever since and has been revealed to be a member of the Great Ring of Arakko -- the ruling body of Krakoa's sister mutant nation. As with the rest of the Great Ring, Tarn is classified as an Omega-Level Mutant, likely thanks to his control over genetics, and has been given a position within Arakko's leadership.
Notably, the other members of the Great Ring show a certain amount of deference and fear towards Tarn. As a newly appointed regent of Arakko and member of the Great Ring, Storm orders Tarn to cease his hostilities against Sinister and, by extension, Krakoa. The rest of the Great Ring concurs, arguing that they are no longer on Amenth and have less to fear of him. Xilo and Lactuca -- who assisted Krakoa in the terraforming of Mars -- remind him that if he were to raise arms against Krakoa that he would be ground into dust. Tarn reluctantly agrees to not attack Krakoa and attracts a wary gaze from one of the Great Ring, Idyll the Future Seer.
It isn't long before Tarn learns his forces have found Sinister, prompting him to break his promise and teleport to Krakoa and Bar Sinister. The mutants of Arakko have been established as ruthless, fearless, and nigh-unstoppable when committed to a conflict. So to see them all so restrained and vengeful in the presence of Tarn hints just how dangerous the dark mutant really is. As the Arakko equivalent of Mister Sinister, he's only been portrayed in a malicious light. But it appears he could be a far greater threat than anyone on Krakoa had really given him credit for -- and his apparent silliness to attack Krakoa could be used by the right party (such as Orchis or Mystique) to cause genuine chaos for Krakoa and Arakko both.