Three times now, I'm using the Summon Spectral Assassin spell and my Spectral Assassin shows up and behaves normally. I also have Lydia as my follower, she has been using a Staff Of Ice Storms. Anyway, often now when my Spectral Assassin and Lydia are fighting the same opponent, my Spectral Assassin will charge after and fight with Lydia, my follower. Lydia keeps killing my Spectral Assassin, and I just find it a bit annoying of a bug. (maybe it's not a bug?)
I think it might be that Lydia might be hitting my Spectral Assassin with the Staff Of Ice Storms, but it seems to only happen when she uses that weapon. Anyway, I just thought I'd post this in case Bethesda sees it, thanks!

