Game concept
Pax Romana Casus Belli
From the game's encyclopedia
A powerful Casus Belli available only to the Ruler of the Roman Empire for the purpose of inaugurating a new era of Pax Romana.
It can target an entire Empire Title at once, and allows for rapid expansion across large swathes of land.
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.
Pax Romana Casus Belli is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A powerful Casus Belli available only to the Ruler of the Roman Empire for the purpose of inaugurating a new era of Pax Romana." It is part of Casus Belli. Patch 1.20.1.2, from game/common/game_concepts/07_ep3_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/07_ep3_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.