Shadow Spetres have been updated in the Doom of My Mymeria FW book. I believe they are now 25 pts. Basically the same with 3+ armour.
I haven't gotten a chance to try this yet, but I have considered it for a long time. The closest I have come is take the Mask of Secrets on the Corsair Prince and some coven units. That already works amazing. I can see that mixing that with the Shadow of death could really hurt some armies and lock them down or break them.
It is a great way to break enemy units on objectives at the end of the game as well