Game concept
Legend Seed
The game also links it as Legend Seeds.
From the game's encyclopedia
A Legend in the making. Legend Seeds are the basis of a Legend and, by founding them, a Ruler may start a Legend.
Legend Seeds can be earned by fulfilling some criteria like being the heir of Charlemagne or by performing legendary deeds: Major Decisions, winning a Great Holy War, becoming a King or Emperor, organizing a Grand Activity, hunting a legendary animal, et cetera.
Inactive Legend Seeds have no cost, but bear no effects. Founding a Legend Seed for the first time has a Gold cost to it.
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 Seed is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Legend in the making." It is part of Legend. The game also links it as Legend Seeds. 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.