Arthur! Monkey Outta Nowhere!

[sector24]

It's Seeking Arrow, an Air 3 spell in the Enchantment tree. It an 8-point armor negating attack on a random commander in a distant provice. Ways to survive it are domes, giving your commanders items (something that grants luck for instance), or a global enchantment like Gift of Health that increases the hit points of all your units. I'm not sure if shields help, but if they do then that's your bet. It probably isn't feasible to craft dozens of items to defend against this, just on your most important commanders.