Hey, a Mirror!Helios that can only say generic death threats is still frickin' scary, man. If it would behave the same way Mirror!Helios did - or could at least behave as intelligently - I think we could still make this work. Giles, despite his protestations to the contrary, does't really suspect Mirror!Helios' presence all that often.

They're basically disembodied hands that come out of the wall from a possessed version of your shadow, drag you by your head into the wall with basically a half second's warning, and dump you somewhere random.