For the new 2011 version, see Claudus (2011 TV series).
Claudus (Sometimes spelled Claudius) is Lion-O's father and the previous Lord of the ThunderCats. Contrary to popular belief, Jaga was not a relative of Lion-O, only a mentor for the young heir to the ThunderCats throne, when they escaped the destruction of Thundera.
Claudus has a yellowish beard and long mane, and a darker skin tone compared to Lion-O. We have a glimpse of how he looked when he was younger in the Shadowmaster episode. Claudus and Lion-O where notably similar in appearance (Claudus had a red mane back then).
In his youth, he used to fight alongside Jaga the Wise, but it would quickly fall upon Jaga to handle the responsibility of carrying the Sword of Omens.
Claudus and Jaga were perhaps the most renowned Thundercats of all time. After Claudus was blinded, Jaga became the main guardian of the Eye of Thundera, the Sword of Omens, and the Treasure of Thundera. It is later stated by Lion-O that Jaga and Claudus were indeed very good friends. According to the series, he ruled Old Thundera some centuries ago (stated due to the ThunderCats lightspeed travel to Third Earth.) He ruled Thundera up until its destruction. He lost his sight during a war he fought against the Mutants of Plun-Darr.
In the episode Return to Thundera, Lion-O is transported back in time to Thundera the day before it was destroyed. No one in Thundera could recognize Lion-O, as he was now a grown-up version of the 12-year-old Thundercub. It was Kano, his old feline pet who recognized him and took him to the chamber where his father was. He saved Claudis from an attack of the past versions of Slithe and Vultureman, who were attempting to recover plans for a Warbot that Claudus' spies had gotten their hands on; he then proceeded to give these plans to Lion-O, who used them in the present to destroy the Mutants' Warbot.
Almost nothing else of Claudus' fate was known for almost all of the series until an ancient enemy appeared to attack the Thundercats back in New Thundera: the Shadowmaster.
Believed to have died on Thundera while assisting the royal flagship in taking off, he was later found to have been captured by the Shadowmaster. When Thundera was about to explode, there was a lunar eclipse in the sky which helped Shadowmaster to increase his powers and open a temporary portal to trap Claudus and take him to the Shadow Realm. The Shadowmaster asked for the help of the Ancient Spirits of Evil if he succeeded in trapping and defeating Lion-O, so that the Thundercats will cease to be a threat to Mumm-Ra and the evil spirits.
Through a sequence of nightmares, Lion-O eventually learns of his father's imprisonment in the Shadow Realm and is able to rescue him. In the same fashion to the previous fight between a Jaga and Claudus team-up holding together the Sword of Omens, here again it is Lion-O who takes the place of Jaga and holds the sacred sword once more with his father. They succeed in defeating the Shadowmaster in his own realm.
Afterwards, Lion-O takes Claudus with him in a force-field barrier made by the sword, but Claudus tells Lion-O that his time has passed and that the true Lord of the ThunderCats is indeed his son, Lion-O, and he is very proud of him. A bit later, after Lion-O explains to Panthro and the rest of the ThunderCats what happened to Claudus, Claudus himself is reunited once more with his great friend, Jaga, both as phantoms.
- We may suspect that, despite the fact that the Shadowmaster entrapped Claudus on a crystal prison, Claudus could not escape from ageing and his body energy was about to expire. Also, we should remember that a similar event happened to Jaga, who, after ageing due to the lightspeed travel to Third Earth, his body became energy and his spirit became a phantom guardian who only Lion-O could see. Now, it is understood that both Jaga and Claudus will observe and help Lion-O in counselling and bond.