In this concept, Grom is a bulky assassin that's good at deceiving enemies and take them by surprise.
Deep Wounds - Trait
His traits defines the way you'll need to play him. He's good at bursting an enemy hero with 3 basic attacks, and go back, or kill him, depending on the situation.
The slow helps in both cases.
Not that it's all nice as a way to clear waves.
The talents are pretty straightforward, you can improve the damage output and the slow, and even make so that it doesn't reset, so that you can keep bashing your target's face.
Windwalk - Q
If you played Warcraft III, you know what this is. This is real fun to play, because it's a good way to escape, to take an enemy by surprise and burst him.
Of course I had to change it a bit to make it more MOBA-like. The duration is really short, but it resets on kill, allowing you to kill your target and get out.
As of now, it also resets on minion kills, which is a really good way to clear minion waves aswell, or you could also use it if the situation changed > kill a minion and use Windwalk again to get out. Do you think this is alright ?
Again, the talents are pretty straightforward, reduced CD, increased damage and charges of block, so that you can trade even better.
Mirror Image - W
If you played Warcraft III, you should also know this one. I'm a bit sad that Nova has something like this in her kit, so I've added something a melee assassin could use : switching place. Cast it in a bush, use windwalk, destroy your enemy and switch.
You could also use it through a wall to escape.
Mirror Images talents work well if you want to deceive your opponents. Reduced CD, Images lasting longer, more Images. If you need survivability, that's the way to go.
Howl of Rage - E
This one is totally new. It allows you to trade well against AA-based heroes, and has a good synergy with your trait. And hey, he's an orc, he doesn't like fireballs, but steel VS steel fights right ?
Howl of Rage talents are good in prolonged fights. You can get increased duration, more reflected damage, but also ability damage reduction if you want to kill that Jaina or KT, and a zone buff for your allies.
Bladestorm - R1
This one isn't new either. It's like Valla's Strafe, but much harder to hit. It's unstopable though, except if you kill him, and synergize with your trait.
With that, Grom is able to dish a bunch of AOE damage.
Plus it'd be an awesome looking ability.
Taking the level 20 talent will allow your images to cause bladestorms aswell. If you want to take that route, don't forget to take the increased duration and the extra Image for awesome AOE damage, and total confusion.
Blood of Mannoroth - R2
I wanted to give the possibility to play as the bad version of Grom aswell, even if it's temporary.
Passively, he's tankier and hits harder, also all his abilities are empowered.
Permanent Fell Grom ? Sure, grab this level 20 talent. Well there's a drawback since your CD will be longer, but hey, they were so short that it wouldn't be fair at all, right ? If you want a tankier Grom for longer fights, take this one.
*His Trait heals him. You don't want to hit multiple targets here, but kill them one by one.
*His Q hits harder and teleports you in the back of your target, dealing lots of damage and activating your trait. You got less escape mechanics, but it's not what you're looking for when taking this ultimate.
*His W summons a Grunt that deals damage, and you can still mess with your opponent mind by switching places.
*His E is just an empowered version of Howl of Rage, that also works on your Grunts.
Heroes of the Storm Build Guides
And btw, I never said it was a signature ability or anything, so I don't really get your point here.
Some feedback and / or advice could be nice though.