Game concept
Nemesis
The game also links it as Nemeses, Nemesis', Nemeses'.
From the game's encyclopedia
A Nemesis is a Special Relation indicating utter (and mutual) contempt and hatred. It is a stronger Special Relation than a Rival. A character can only have one Nemesis.
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.
Nemesis is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Nemesis is a Special Relation indicating utter (and mutual) contempt and hatred." It is part of Special Relation. The game also links it as Nemeses, Nemesis' and Nemeses'. 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.