Game concept
Short Reign
From the game's encyclopedia
'Short Reign' is a decaying Opinion penalty that all characters in a Realm get with a new Liege.
In a similar way, subjects get an increasing Long Reign Opinion bonus with Lieges who have ruled over them for a long time.
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.
Short Reign is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "'Short Reign' is a decaying Opinion penalty that all characters in a Realm get with a new Liege." 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.