I'd say to take small steps to find the balance YOU want. It's your Mod, so at the end of the day you're the one that needs to be happy with it.
You obviously modeled Duke after the "Great Wolf " unit in-game.
Biggest Problem IMHO
"Fear" has the (high) chance to stun all enemies for 3 turns unless they resist, combined with Wolf Howl becomes OP
There are a couple of ways you could replace the Fear Trigger, depending on your original idea with Duke
1. Give Duke a Permanent Cloak of Fear (as per spell). Downside, the effect only applies to Duke.
2. For an AoE effect, you could give Duke the Pandemonium Spell to simulate the original intent you had with Fear. Not sure if it could be set to trigger on entering combat like Fear does? Only drawback with Pandemonium (For Duke) is the 1 Turn casting time. If the casting time could be reduced to 0 it would be good to go. If it would be a pain to create a Triggered Pandemonium you could give him Water Adept III from the Henchmen Path. Using the normal Water Path would seem to go outside your concept.
3. Similar to #2, Use Wither or other AoE if Pandemonium seems wonky
Another potential problem comes from "Pack Tactics" existing in the Warrior Tree. Savvy players will pick the Warrior Path for instant access to all the goodies that follow afterwards. This might be mitigated by locking Duke into the Assassin Path. Would probably be a last resort after dealing with the Fear issue above. Personal note: I would really miss Blade Rush if this were to happen.
Add: Set Base Cold Resistance to 25: Per your flavor text, and really only adds some flavor as being hit with Cold Damage is pretty rare
CounterAttack: Obviously needs to stay, but could be changed to Reprisal which I don't think alters Duke's power all that much in the scheme of things. Would depend on personal preference, and how you work out the other balance issues.
Creature Type: Beast; This could be an issue as I think Duke could be targeted by the Beastmaster Sovereign or the Beast Control Collar in Combat. I'm not sure there's any real solution to this.
[I may edit this later after thinking on it more]