Game concept
Imperial Expedition
The game also links it as Expedition.
From the game's encyclopedia
An extremely powerful Casus Belli usable by Co-Emperors in Duumvirates.
An Imperial Expedition may be launched against any Realm that either borders the Empire or holds lands that De Jure belong to the senior emperor.
It can (and generally will) target all Duchies that de facto border the Empire or De Jure belong to it.
Further, every Governor bordering the defender will be forcibly called to War as an Ally. Remote Governors with military Province Administrations may optionally join the conflict.
If the War is won, the Co-Emperor must decide between:
• Turning the conquered lands over to their senior emperor for Influence and Opinion
• Keeping the territories for themselves (costing Opinion)
• Handing over most of them whilst keeping the best Duchy (slightly reduced Influence and Opinion)
• Appointing local interim Governors who will be indebted to them
All rewards scale to the land taken: a large conquest earns significantly more Influence than a small border war.
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.
Imperial Expedition is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "An extremely powerful Casus Belli usable by Co-Emperors in Duumvirates." It is part of Casus Belli. The game also links it as Expedition. 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.