You get a war to take over a specific spot. If you win then it gives all your counties to randomly generated guys and breaks the Duchie title (if you had one). It's a really good way to get onto any given feudal territory.
If you do it from Scandinavia you get additional man at arms via event troops. Given these are all Norse unique units they are very very powerful.
You can do multiple of them as long as you're less then a king, norse, and its the tribal era. Though they get more expensive each time.
Really good way to abuse this is to launch one at the start to wherever, then conquer a relatively easy county somewhere else, then use that to slingshot your diplomatic range so you can adventure wherever you really want to be.