Game concept
Forced Vassalization Casus Belli
The game also links it as Forced Vassalization, Forcefully Vassalized.
From the game's encyclopedia
A Casus Belli allowing for the forceful Vassalization of a neighboring independent Ruler of lower rank. Unlocked by the Forced Vassalage Perk.
You can Vassalize any Ruler that does not have more than 4 Counties in their Realm.
This limit is increased by +3 for each of the following innovations:
• Casus Belli
• Chronicle Writing
• Divine Right
• Rightful Ownership
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.
Forced Vassalization Casus Belli is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Casus Belli allowing for the forceful Vassalization of a neighboring independent Ruler of lower rank." It is part of Casus Belli. The game also links it as Forced Vassalization and Forcefully Vassalized. 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.