Declaration of War is a quest item in Skyrim: Dragonborn
Raven Rock Stronghold,
My calls for the unconditional surrender of your forces and an immediate cessation of all hostilities has been ignored numerous times. I therefore have no choice but to assume your purpose on Solstheim is hostile, and treat Raven Rock Stronghold as an enemy to the Empire. I warn you, any attempt to breach Fort Frostmoth will be met with an equal level of aggression. I will do everything in my power to wipe you and your forces off the face of Tamriel. There will be no further communication between us.