Succession law change for one kingdom affects all vassals everywhere.

[Lessing]

I usurped the kingdoms of Mauretania and Africa and then changed the respective succession laws from Gavelkind to Primogeniture, and from Agnatic to Agnatic-Cognatic.

The duke of Iceland (and in fact each of my direct vassals) got an "upset by succession law change".

Now, why would the duke of Iceland care about the succession law change on the other end of the world in another de jure kingdom, especially since I am changing the laws of Mauretania and Africa to what it is in Iceland in the first place: Agnatic-Cognatic Primogeniture.

 

[K-vald]

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