ThunderCats wiki

The Moon*Star is a satellite located near Mon*Star's base of operations. It radiates a wavelength of radiation, unknown to Earth science, that will cause Mon*Star to transform whenever he is exposed to it. Yes-Man, a sapient, human-sized worm with arms but no legs, usually controls the bombardments of the Moon*Star; on one occasion, Hardware infused the Moon*Star's radiation into a specially-designed box that he then smuggled to Mon*Star.

Whenever he decides to involve himself directly in fighting against the SilverHawks, Mon*Star subjects himself to the radiating light from the Moon*Star to assume his armored form while reciting the following chanted order:
"Moon*Star Of Limbo...
give me the might... the muscle... the menace--
--of MON*STAR!!!"