Game concept
Legend Library
From the game's encyclopedia
The Legend Library holds completed Legends that have spread to your Domain, you created, or were actively promoting.
You can also actively add completed Legends if you are traveling through the spread area of the legend.
You can remove Legends from your library at will.
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 Library is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Legend Library holds completed Legends that have spread to your Domain, you created, or were actively promoting." It is part of Legend. 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.