I was always told that as assassin bugs eat other bugs, if you see them in a large group they are not assassin bugs; some immature types of stink bugs look a lot the same. That would be my guess. My other guess is that the section of the tree pictured is dead and will probably interfere with the transport of water and nutrients up and down. I would not be surprised if the branches on that side above the scar did die. Ask the experts to be safe.
