Ferit was one a fairly strong option because of his full mana drain combined with above average single target damage with the drawback of not being able to target. I used him a lot in early game PvP. Options available now make him a joke in terms of damage output even at early game, but mana drain still scales infinitely.
Overall, I think he could use a buff. Thematically, his spell should have greater effect on daemons. Although he drains, not being able to target it and hitting the last troop, who usually fills last, has limited usefulness unless you are in a one on one situation. This was way more common in the old days, but not so much now even at early game.
I’d give him give a slight base boost in spell damage and triple damage and extra turn conditionals. Raise mana cost a bit to compensate. Accordingly:
Backstab (12 Purple/Brown)
Deal [Magic + 8] damage to the last enemy, and drain their mana. Deal triple damage if they are a Daemon. Gain an extra turn if the enemy dies.