Shadowdermis is a mysterious virus with vast power and the ability to do almost anything. However, making it do so requires large amounts of energy, not to mention patience. Most beings do not know of its power and use it to interrupt dimensional portals (which may be overrided by great enough strength of will), as well as for the purpose of mutation.
It was created by Makuta Kunaku in a failed attempt to replicate the properties of Energized Protodermis. Kunaku discovered its power and used it, along with Hordika venom and Pit mutagen, to create a monstrous Rahi called Mephiles. Velnax, a Kodax posing as a Makuta, saw its potential sent it to Gigas Magna for it to aid in Forsk's rebellion. The Threen were engineered to be able to inject Shadowdermis into their victims.
Tetrack attempted to use Shadowdermis during the Underworld War to root out the Gigas Magna Resistance. What he used it for is unknown. He also began a project to create a new, improved form of Shadowdermis, which bore fruit a very short time before the end of the Underworld War. The project was kept so secret that only Tetrack and the beings working on it knew of its existence.
At the beginning of the First Order of Darkness War, Kunaku released it on the entire island of Xa Nui, infecting all its inhabitants and turning them into war drones for the Order of Darkness. The infected Xa-Matoran helped in many battles, most notably the Capture of Sacho Nui.
Fairon later destroyed the Shadowdermis' source, a pool beneath Xa Nui, and annihilated it. All Shadowdermis in all the universes was immediately annihilated, with the exception of Tetrack's experimental substance.