From the game's encyclopedia
A Dynasty Head may denounce a member of their Dynasty for a cost of Renown.
Any member of the same dynasty (including the Dynasty Head themselves) will gain an Imprisonment Reason against the denounced member.
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.
Denounce is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Dynasty Head may denounce a member of their Dynasty for a cost of Renown." It is part of Dynasty. The game also links it as Denouncement. 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.