Game concept
War Chest
From the game's encyclopedia
The War Chest is a collection of Gold, Piety and Prestige. The contents are primarily used as rewards for successfully winning Undirected Great Holy Wars, where they are divided between the attackers based upon their participation in the War.
A certain portion of what's in the War Chest is also divided between the attackers as the War is declared.
If an Undirected Great Holy War is defeated, the defenders will divide the Piety and Prestige portion of the War Chest between them (the Gold is lost.)
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.
War Chest is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The War Chest is a collection of Gold, Piety and Prestige." 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.