After Toxium destroyed a nuclear power plant from a foreign country, it decided to create an army of toxic creatures by using the leftover Uranium from the power plant in order to expand its territory that it invaded and later further. Just as Toxium and its army are about to invade a city as part of its land's expansion, the military who guards the city immediately prepared themselves on defending their city by shooting down the toxic creatures before they cross the border. Most of the toxic creatures were killed but Toxium and the others survived as they escape the city by riding an abandoned airplane, which they activated it using Uranium. Toxium then vows to get revenge on those who live in the city it tried to invade for the loss of the majority of its army. They fly to another country to find another nuclear power plant, steal its Uranium and create more armies of toxic creatures, with the help of its allies. But when they encounter another city, they decided to invade it before another military shows up. Mario and Kirby, who're just hanging out, noticed what Toxium's doing and confronted it along side with its army. 

Toxium is a slime like toxic creature made out of Gallium mixed with Uranium. It was created when a Gallium accidentally got spilled from a container to a Uranium barrel that was left open, which caused the Gallium that went inside to mutate with the Uranium inside it and eventually evolved into a living toxic creature. It also gained intelligence as it was able to destroy the whole power plant by overheating the reactor and make its own army by using the leftover Uranium. Toxium won't be in the Kirby and Super Mario franchise, as it'll be in its own movie or TV series. 

Kirby and Mario belong to Nintendo.

Toxium belongs to me.