Radon

Radon the heaviest and at the same time the only radioactive noble gas. It is a decay product of naturally occurring uranium or thorium and dissolves out of the ground into the atmosphere, where it aggregates in caves or cellars. Even though the element is radioactive, it is supposed to have healing qualities. There are even official spas that offer radon baths as a form of therapy. The medicinal benefit is disputed.

Radonis the monster is so heavy, that it can barely stay above ground with its small propellers. Because of its radioactivity it is an underdog amon the noble gas monsters. Repeatedly the lords of the underground Urano and Pluto are trying to make Radonis switch to their side.