You need to search for clues in the Old Attius Farm, as to why the Ash Spawn are attacking. Check out the Ash Piles on the ground. One of them is lootable. A quest marker should lead you to its location. From there, you loot a Declaration of War. Read the note, then give the note to Captain Veleth. Tell him you discovered this note on one of the Ash Spawn.

Veleth says that this is strange, the note says it's from General Falx Carius, but that's impossible  because General Carius died 200 years ago and that there's no way he could still be alive. Ask Veleth what he wants you to do. Veleth tells you that if General Carius is still alive, there must be something keeping him that way. You need to head out to Fort Frostmoth and check it out. Captain Veleth will head bck to Raven Rock and prepare his men for further assaults.

Your next objective is to kill general Falx Carius. Veleth warns you to be careful, this General Carius sounds out of his mind. Time to head over to Fort Frostmoth to see what's going on. The Fort is located north east of the Attius Farm, see the map below for help finding it:

Fort Frostmoth in Skyrim Dragonborn

Watch out for hostile creatures on the way, such as the Ash Hopper.

There are some points of interest which you can explore on the way, including Hrodulf's House which is between the farm and the fort, or the Wreck of the Strident Squall, a shipwreck on the shore near the farm.

The journey to Fort Frostmoth isn't that long, and soon you arrive there.