Game concept
Challenger Head of Faith
The game also links it as Antipope.
From the game's encyclopedia
A Challenger Head of Faith is a rival Head of Faith raised against the Mainline Rite Head of Faith.
A Ruler may sponsor a member of their Clergy as Challenger Head of Faith. A Clergy Ruler or a ruler from a Faith with a Temporal Head of Faith may declare themselves as a Challenger instead.
A sponsored Challenger Head of Faith takes up their sponsor's Rite, becomes that sponsor's Clerical Ruler Puppet and owes them a Weak Hook.
The Mainline Rite Head of Faith cannot Excommunicate anyone who follows a Challenger Head of Faith: the sponsor and their Vassals.
Everyone following a Challenger Head of Faith brings their clerical requests to them instead of to the Mainline Rite Head of Faith, among them these Interactions:
A Challenger Head of Faith or their sponsor may use the Challenge Head of Faith Casus Belli against the legitimate Head of Faith.
Rival Kings or above of the same Faith may instead use the Depose Antipope against a sponsor, deposing the rival claimant.
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.
Challenger Head of Faith is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Challenger Head of Faith is a rival Head of Faith raised against the Mainline Rite Head of Faith." It is part of Head of Faith. The game also links it as Antipope. Patch 1.20.1.2, from game/common/game_concepts/pam_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/pam_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.