Claudus was a fierce king who was much loved by his subjects and feared by his enemies. As lord of the Thundercats, he maintained order in the lands of Third Earth by keeping the Cats in a position of power and repressing the other animal races, most notably the Lizards. He wished to pass on leadership of the Cats to his adopted son Tygra, but tradition demanded that the crown be passed down to the blood prince Lion-O, whom Claudus felt was too immature to take up the mantle of king.
In the second episode, "Ancient Spirits of Evil", Thundera was invaded by the Lizard army, who had acquired previously unheard-of technology and used it to overwhelm Thundera's defenses which were primitive by comparison. Claudus fought against his treacherous former general Grune for the life of the captive Panthro and managed to free Panthro and defeat Grune with the help of his sons Lion-O and Tygra. However, the captive Panthro was not Panthro at all, but actually the evil Mumm-Ra in disguise. Mumm-Ra drew a knife and stabbed Claudus in the back, sending the king plummeting from the top of the mound in the center of the city arena. As Claudus lay dying in his son's arms, he told Lion-O that he had made him proud.