An Unfortunate Accident
Rare9.6%ObjectiveOptionalSingle-player
Inherit a title from someone you murdered
Be someone's heir, murder them, inherit. Ibrahim ibn Ahmad, Duke of Gabes (867) is the ready-made setup: he's heir to his brother, the Aghlabid ruler. Check the succession panel to confirm you're primary heir, start the Murder scheme, pad it with high-intrigue agents who dislike the target, and when it lands the title passes to you along with the achievement.