In my opinion this is the best Siege creature out there. With the latest update most Territories only have 2-4 HP. 4 Pwr will easily kill most of them.
Also Shadow makes it possible to sneak to the Territory you want to get rid of!
And if it gets blocked it will hit the enemy creature with a high chance, cause of the 6 Atk!
So as an offensive unit this is a 6/5 stars, but it won't live very long, when you use it as a Blocker.
Maybe hard to get, because you need to hunt the Achievements, but once you got 4 copies, go for the Keb-rush!