Hotaru

When rising sea levels lead to the extinction of humans, the apex of the mammalian order, and the subsequent demise of terrestrial life, a race of squid evolves that can inject an ink-like substance into their bodies, giving rise to the Inklings, a race that can change from squid form to humanoid form at will. While the squids were evolving, their cephalopod cousins, the octopuses, evolved a similar form called the octoring. The Inklings and Octorings use these inks to fight territorial battles, and the Inklings have remained alive to this day, with the Inklings being the more aggressive and the Octorings the more engineered. After a great war with the Octoring, the Inklings took over the surface and drove the Octoring underground, where they continue to entertain themselves above ground with no thought for the future with things like Territory Battles. The Octorlings, now called Octorians, are working tirelessly to reclaim the surface, though some have managed to reach the surface by some means, and the Inklings have forgotten the past and are coexisting with them.