Just go along with it. They needed to show the audience that Grommash, despite being a mass-murderer who committed genocide upon the Draenei and Stormwind and surrendered himself willingly to the Legion twice, was not evil anymore and joined the good guys.
Grommash Hellscream was high on Demon Juice, making him that much more powerfull. He was already known for taking down Cenarius! On the otherside I assume Mannoroth never assumed Grommash would turn against him, thus never going full power against him and seeing him more like a rabid dog who had to be put back in its place, I suppose? And maybe Mannoroth was weakened, I dunno man…
Anyhows, Grom got what he deserved and both the MU and AU Grommash are mass-murdering maniacs who never deserved the name of hero.
Even still, It is demon-made weapon in the hands of a demon that’s at least 20 times Grommash’s size. Comparing a mountain buster to a single orc.
All the odds are stacked against Grom.
You want to fight at least 20:1 and rising zombies at once that can be raised again?
Gorehowl was forged three generations before Grommash inherited it. Grommash’s great-grandfather used Gorehowl to kill six legendary gronn. Their hearts were sealed into the blade, granting it untold strength.
Woah, thats new information for me! So its an true Orcish artifact now, alongside Doomhammer? Kinda sweet and a nice touch to the “just a normal Axe who makes a weird sound when you use it” history we had before that!