Game concept
Exceptional Epic
From the game's encyclopedia
A Family Epic written through the Commission Epic Decision has a chance of being an Exceptional Epic, which confers greater bonuses than a normal epic.
The chance of the Epic being Exceptional can be affected by event choices.
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.
Exceptional Epic is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Family Epic written through the Commission Epic Decision has a chance of being an Exceptional Epic, which confers greater bonuses than a normal epic." It is part of Commission Epic. 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.