Game concept
Mongol Invasion Casus Belli
From the game's encyclopedia
A powerful Casus Belli available only to the Ruler of the Mongol Empire.
It can target an entire Kingdom 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.
Mongol Invasion 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 Mongol Empire." It is part of Casus Belli. 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.