The best ever episode of Thundercats
From season 4; episode "Malcar".
Plot summary: Mumm-Ra summons Malcar, an ancient alchemist, to return from the dead. The alchemist returns old and weak, as he was when he died. Mumm-Ra takes him to the canyons of youth to restore his youth and power. Malcar regains his powers, and uses them against the Thundercats. He transforms all the Thunderillium on the planet into Thundrainium.