WARNING: The following contains spoilers for Planet-Size X-Men #1 by Gerry Duggan, Pepe Larraz, Marte Gracia & VC's Clayton Cowles, on sale now.
Arakko was the lost mutant nation, the sister island of Krakoa that was forced -- along with its warlike and powerful mutant inhabitants -- to venture into the realm of Amenth. The Swordbearers of Arakko challenged the heroes of Krakoa in X of Swords and were defeated, but in an act of peace, the X-Men and their allies arranged for Arakko to return to Earth and live along Krakoa in peace. However, the mutants of that land had only ever known survival of the fittest.
As explained by Isca the Unbeaten when she met with Xavier and Magneto, all mutants who thrive on the island and sit among the Great Ring of Arakko -- their equivalent of the Quiet Council of Krakoa -- are Omega-Level mutants. This means that they have almost no comparable limit in terms of power and capability. Now, three of them were just used for the X-Men's most ambitious plan in years.
Three of these mutants end up being recruited by Magneto and his allies on Krakoa for a very important mission. Describing their powers as "weapons," Isca and the mutants of Arakko agree to work with the figures from Krakoa to achieve their near-impossible mission. Working alongside Krakoa's Omega-Level mutants Magneto, Hope, Vulcan, Storm, Iceman, Marvel Girl, Exodus, Elixir, Quinten Quire and Monarch, the three are able to effectively terraform Mars and transform the long-dead red planet into a world for mutants to flourish. They even bring the mutants of Arakko straight to the planet by moving the External Gate so that it drags Arakko exactly where it needs to be.
Each of the three selected Arakko mutants ends up utilizing their powers in synch with their Krakoa allies. Sobnuar of the Depths is a tall figure, with a toad-like face and six fins across the side of his head. Working with the power-amplifying Hope and the reality-warping Monarch, Sobnuar is revealed to contain an entire aquatic ecosystem within his veins. Making a cut across his hand, Sobunar pours water onto Mars and gives it oceans new life. Storm then assists by using her control over weather to create new natural winds for the world, allowing her to marry the land and sea of the world in a natural ecosystem.
Xilo, First Defender of this Broken Land, is a large beetle-like mutant. With two large golden antenna and round eyes, he effectively appears to be an eight-foot-tall insect. Reaching Olympus Mons -- the newly mutant-created active volcano on the surface -- Xilo effectively transforms into hundreds of thin strands that break into the Martian ground and help spread elements into the land. The oxygen he creates allows the mutants to breathe in the world.
Finally, Lactuca appears to be a female figure wearing a black cloak over her entire body. In reality, this "cloak" appears to be part of her and is seemingly filled with stars. Her ability is to know everything in the universe -- including when to conduct dangerous plans so that they may work. In essence, she seems to have a forward-thinking variant of Domino's powers, allowing her a surprising mastery over the universe. It's overall a very impressive showing from Arakko and allows them to help claim the credit for transforming the dead world while also establishing their power to the rest of the watching world.