Ironfur
Quest: While in Bear Form, gathering a Regeneration Globe or participating in a Hero Takedown increases your Spell Armor by 0.1 per second, up to 5 and your Physical Armor by 0.2, up to 10.
Reward: After gathering 50 Regeneration Globes or participating in a Hero Takedown, Demoralizing Roar, Thrash, and Wild Charge have their cooldowns reduced by 0.5.
Tooth and Claw (W2)
Thrash increases the damage of Hamuul's next Thrash by 5% and reduces the cooldown by 0.5 seconds, stacking 6 times.
Lasting Spines
Thorns Aura deals 200% additional damage, but deals its damage 100% slower.
Quest: Damaging Heroes with Thorns Aura increases it's damage by 10, up to 500.
Reward: Hamuul now deals 18 (plus 4% per level) damage whenever he is hit, increased by 33 in Bear Form.
I love the focus and synergy around applying the curse and the bonuses you get from it, while particularly massive on the ranged assassin/druid form, makes perfect sense as it takes a LOT of effort to keep those bonuses active. But despite this the combos feel natural and flow well. I could see a good Hamuul player swapping between forms whenever possible to get as much value as possible from the curse application.
While the base abilities themselves are fine, i think ironfur's numbers need a bit tweaking. Having passive 15 armor in both forms seems really strong seeing as no other heroes get 15 armor baseline(except for magical armor only). Perhaps make it only give the additional armor in bearform, and only have the regen orbs count if he picks them up in Bear form?
Also i would move Nourish to a level 20 talent as its way too strong as a level 1 talent. That healing boost and healing it provides in general rivals some actual support heroes. Perhaps replace tooth and claw as tooth and claw is kinda on the weaker side of the level 20 talents?
Seriously though, i just love this concept and its an utter shame its not more popular. You clearly put a lot of effort into this, and I salute you. Maybe you could help get more attention if you changed the character to a more lore-important bear druid? Maybe boll Bearmantle? People tend to look at recognizable characters more than little-known side ones.
But if you do want to keep it as hamuul, perhaps add that doomguard hamuul should maybe turn into a fel hound instead of a bear when he transforms? Would fit the theme of that skin more.