Black Rook Missive

The Black Rook Missive is a reward from the rare class hall mission Investigate Black Rook Hold. There are three identical versions for Normal, Heroic, and Mythic difficulties.

This item starts Black Rook Hold: The Lord of Black Rook Hold.

While it turns up from time to time in your mission queue, it will probably be quite some time before you'll get your followers' ilvl high enough to actually do it.