Usurpation
If a character is holding more than 50% of the De Jure Counties of a Title held by a Ruler of the same Faith or a Faith which is not considered Hostile or Evil, then the character can Usurp the Title. If the holder's Faith is considered Hostile or Evil, then Usurpation requires the character to hold all De Jure territory of a Title.
If a character is a Vassal, they cannot Usurp a Title belonging to the Liege, nor can they Usurp one that has the same Rank or higher than the Liege's Primary Title.