From the game's encyclopedia
Dynasty Legacies are unlockable boons that provide powerful effects to all members of a Dynasty.
The Dynasty Head unlocks Dynasty Legacies by spending Renown, with each Legacy in the same track being progressively more expensive and powerful.
Dynasty Legacies are slow to get, but stay with the Dynasty forever.
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.
Dynasty Legacy is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Dynasty Legacies are unlockable boons that provide powerful effects to all members of a Dynasty." The game also links it as Dynasty Legacies. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.