I was going to make a Marrowgar, but then I see this shining gem and stand down. Great job on this character, I would love to see him in the game.

Broll was a very rage driven person, so it seems unfitting for him to only have one damage ability. I could see Demoralizing Roar being replaced with something else that shows his strength. Also, Broll was forsaken by all Spirits but the Bear, so Bear form is his only accessible form, if I'm not mistaken. This is due to him losing himself to rage at the Battle for Mount Hyjal.

Broll Bearmantle can't access Cat Form, actually. He was abandoned by all Spirits but the Bear, only allowing him to access Bear Form.

I think having a 3.5 second AoE fear as an ability is over powered. 1.5 seconds with no scaling would be more in line with the balance of the game. Dark Portal sounds insanely silly, the Demons last far too long and by being immune to magic practically makes killing them impossible for a lot of Heroes. The main issue with this is that it isn't cool or anything, it's unfun. Though, I don't know their health pools, but taking more than 3 basics each would be about the point where I'd dislike...

700% seems fine since it's hard to meet the conditions and the base is VERY low (210 at Lv. 20). It's also close range on a squishy squish. I wouldn't count it as an auto attack though, except for the purposes of his passive.

My main point of worry would be that Bone Splinter does more damage than Nova's Snipe and I imagine it having similar functionality (line target). More base, 1 more point scaling, and can store two charges and have damage increases through talent optimization. My main complain would be that the second Heroic sounds really bad, as not only do you have to get A LOT of stacks for it to matter, but you have to do it each time for it to matter and it puts you back into a bad situation with 40%...

Sinister Strike does A LOT of damage, and should probably be adjusted. Fan of Knives being a chance to slow is silly, even if it's trying to replicate Crippling, chance effects are unreliable and don't add fun but luck. Cloak of Shadows is odd as well, it should just make you immune to abilities for a duration and maybe decrease damage taken, as opposed to a by far worse Evasion. Ambush simply promotes a bland assassin that waits for ultimate to burst efficiently. Really, the problem I have...