A Title held by a foreign Ruler can usually be Usurped if more than 50% of tits De Jure Counties are within your Realm. This process gives the Title to you, which is particularly important for ensuring you are the Rightful Liege of your Vassals.
Usurpation is more difficult if the current title holder’s Faith is hostile to your faith. In this case usurpation is only possible if the title holder no longer possesses any of its De Jure Counties inside his/her Realm.
In the video above we walk you through an example of usurping a Duchy Title for our own. In the example video we are a County Ranked Realm, Daura and are usurping the Duchy title of the High Chiefdom of Kebbi. There are three (3) County Titles within the High Chiefdom of Kebbi; Chiefdom of Sokoto, Chiefdom of Yauri, and Chiefdom of Kebbi. In this case two of these Counties fall inside my realm giving me the 51% control of Counties within the title. Since we are of the same faith in the video, the requirement of the title holder having no Counties in the title was not needed.
Keep in mind that when usurping a Title you will need to pay gold to do so. In this case it was 250 gold to usurp the Title of the High Chiefdom of Kebbi. In return I gained the title and 300 Prestige. Usurping does not take over the land that the Title holds.