From the game's encyclopedia
A Casus Belli that allows a House Head to reclaim House members from another Realm as Vassal.
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.
Unify The House is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Casus Belli that allows a House Head to reclaim House members from another Realm as Vassal." It is part of Casus Belli. Patch 1.20.1.2, from game/common/game_concepts/03_fp3_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
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Source: game/common/game_concepts/03_fp3_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.