Are devils worth 7 blood each?

[Cohen]

You need too some research in Thaumathurgy.
It casts a Soul Slay or Mind Burn (the better you've researched beetween those 2), and cast it remotely (those spells are combat spells) on an enemy commander in that province.
It checks on MR and if fails to pass the check the enemy die if you've Soul Slay.
I know this strikes too hiding unit and glamouring ones. (I had a Man Thuatha besieging one of my castles ... the map shows that Man was besieging me but no units were shown ... I went for Mind Hunt, the spell found the target but sadly failed to kill him).
But it has a very high double edge... if there's an astral mage in the target province this will feeblemind all your guys casting the spell in that province (it's not very realistic imo that a single guy 1S could feeblemind 6 S3 or S4 mages casting that spell in the same turn). In fact this spell is usually used by Arco, that has healing priestess and cheap astrologers.