Game concept
Legend Quality
The game also links it as Famed, Illustrious, Mythical.
From the game's encyclopedia
The Quality of a Legend marks both the strength of its effects and the reach of its spread, as well as affecting its final rewards. Legends can be Famed, Illustrious or Mythical — for there is no such thing as a common Legend.
The Quality can be increased by spreading the Legend to more Baronies and paying an upgrade cost.
When the owner of a Legend dies, their family can continue it, with no damage to its Quality.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Legend Quality is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Quality of a Legend marks both the strength of its effects and the reach of its spread, as well as affecting its final rewards." It is part of Legend. The game also links it as Famed, Illustrious and Mythical. Patch 1.20.1.2, from game/common/game_concepts/06_ce1_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/06_ce1_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.