Game concept
Migrate to Pannonia Casus Belli
The game also links it as Migrate to Pannonia.
From the game's encyclopedia
A powerful Casus Belli that may only be used once per campaign, total, to conquer all territories held by the target within Pannonia.
On victory, all Mogyërs in the attacker's Realm will move with them to Pannonia, Counties and Vassals, displacing Avar Counties. Other vassals outside of Pannonia are abandoned, while Counties formerly occupied or held by Mogyërs are left to local petty warlords and rival steppe tribes.
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.
Migrate to Pannonia Casus Belli is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A powerful Casus Belli that may only be used once per campaign, total, to conquer all territories held by the target within Pannonia." It is part of Casus Belli. The game also links it as Migrate to Pannonia. 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.