Sequid

Each individual Sequid is a small pink creature that resembles a squid, trailing a trio of tentacles beneath an uncovered head shaped like a naked brain. On its own it looks almost harmless, which masks how devastating the swarm becomes once a single one finds a body to inhabit.

A shared mind binds the swarm, letting the creatures behave as a single organism rather than a crowd of separate animals. That unity only ignites when one of them seizes a living body, and a lone possession is enough to snap the entire colony into coordinated action. Until that happens the individuals drift about aimlessly, weak and obedient, which is why the Martians once put them to work as docile servants. Even in that servile state they never stop grasping for a host, latching onto anything alive in the hope of merging with it.

With a body under their control the Sequids grow powerful, focused, and merciless, tearing through whatever stands before them as though ruin were the goal itself. Their bottled resentment showed when they turned on the Martians who had subjugated them for so long. They also remember past foes, recalling that Invincible had resisted possession before and choosing to kill him rather than convert him, proof of a calculating streak. For all their arrogance they can plan patiently, hiding and waiting for a fresh host when a scheme collapses. Their true flaw is habit, since they keep repeating the same tactics, which makes them predictable and far from adaptable.

Long before the series, the swarm carved a path of destruction through the cosmos, hopping from host to host until it crashed onto Mars. There the Martians proved impossible to infest, because their shapeshifting bodies would not let a Sequid break through the skin. Unable to seize their would-be masters, the colony was reduced to a scattered, powerless labor force serving the very race it could not conquer.

Everything changed when a crew of astronauts arrived from Earth with Invincible quietly assigned to guard them. A Martian had secretly taken the identity of astronaut Rus Livingston, leaving the real man exposed and giving the Sequids the host they needed. Newly unified, they overran Mars and boarded a ship bound for Earth. The Guardians of the Globe intercepted them, but the moment Bulletproof killed one, the swarm sensed the loss and closed in on the team. Atom Eve was knocked cold and Shapesmith wrapped the group in a protective shell while Robot built a device that scrubbed the parasites out of Rus. Eve and Robot were briefly taken, then freed by the same device, and with no bodies left to ride, the threat seemed over.

One survivor, though, rode home inside Rus's gut and later crawled free to claim him again. After the Invincible War it bred in the sewers and finally poured into the streets hunting fresh hosts, only to meet a citywide containment grid, cure-firing disruptors, and teleport darts prepared by the Guardians and the GDA. As their equipment failed, Invincible caught the possessed Rus and, with the barrier about to collapse, made the brutal choice to kill him so the infection could not spread. The cost lingered afterward, as Robot coughed up one last hidden Sequid in the showers and Monster Girl destroyed it before it could take him, closing the outbreak for now. Notably the parasites can never touch Viltrumites, whose skin they cannot pierce, nor Martians, who simply expel them.