Lord of the ThunderCats is a hereditary title bestowed upon a member of the Thunderian Royal family who will go on to lead the ThunderCats. The title has been passed down from father to son for generations. The lord also is given the Sword of Omens to safeguard as well as use as a weapon since the sword contains the Eye of Thundera the source of the ThunderCats' powers.
Claudus was the first Lord of the ThunderCats that was shown on the cartoon show. He held the title while the ThunderCats still lived on Thundera. Then after he lost his eyesight fighting with the Mutants, Jaga was entrusted the task of leading the ThunderCats and safeguarding the Sword of Omens. Jaga then handed over the sword to a young Lion-O when the ThunderCats were fleeing the explosion of Thundera in which Claudus was assumed to have died.
Lion-O remained the hereditary Lord of the ThunderCats on Third Earth until his 20th birthday when he had to participate in the Anointment Trials in order to prove that he truly is worthy of his title. After passing all five Anointment Trials, Lion-O was crowned the true and legitimate Lord of the ThunderCats.