The Legacy Stones are twin gems of ultimate power. Whoever unlocks the power of the stones by saying " Mighty stone legend old, power unto me unfold." will become all powerful and unstoppable.
He-Man and Skeletor both activated one of the stones and gained a super-powerful form, but after they battled for a while He-Man realized they were too evenly matched for one to ever defeat the other. He instead shifted down from his Legacy Stone-enabled form to lure Skeletor into attacking him, during which he stole Skeletor's stone too, and then destroyed them both.