Game concept · All Under Heaven
Son of Heaven
The game also links it as Son of Heaven's.
From the game's encyclopedia
The Son of Heaven is the Hegemon of China and the Top Liege of the Celestial Government.
The Son of Heaven has a special form of Legitimacy, the Mandate of Heaven, and a unique Participant Group simply called Son of Heaven.
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.
Son of Heaven is an entry of Crusader Kings III's in-game encyclopedia (All Under Heaven). The game explains it: "The Son of Heaven is the Hegemon of China and the Top Liege of the Celestial Government." It is part of Mandate of Heaven. The game also links it as Son of Heaven's. Patch 1.20.1.2, from game/common/game_concepts/tgp_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/tgp_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.