The result of his life-draining power on Durotan is not pretty to see. In fact, it's so horrifying that Orgrim Doomhammer is then able to drop an Armor-Piercing Question by asking if the orcs want to follow a "demon" like Gul'dan, leading to the rest of the Horde (albeit momentarily) turning on the warlock.
Several Frostwolf clan orcs are slaughtered and crucified to show what happens when you defy Gul'dan.
Blackhand getting his left hand mutilated when he grabs a loaded gun by the barrel is disgusting. When Gul'dan starts to slowly disintegrate him from that hand up, something only Durotan's intervention saves him from, it's worse. The truly creepy part comes when Gul'dan imbues Blackhand with the fel; the distorted, blade-fingered prosthetic he's taken to wearing by that point fuses to his arm, spider-webbing of green flesh crawling over the metal for a horrific, half-rotted look.
Medivh's face as the fel takes over him. Black Eyes of Evil, withering, flaking skin, extremely prominent veins and a "beard" of jagged spines that grotesquely thrust themselves out of his chin.
Then finally there's Medvih turning into a literal demon monster that appears almost 25 feet tall and he's still reciting his ominous chant. It's a miracle Khadgar crushed him with the clay creature he's brought to life, otherwise Lothar would have been seriously screwed.
If you think for a moment and remember your lore, that scene gets even worse: that was basically Sargeras taking full control of his host...
The first fight between the orcs and the humans showcases just how terrifying the orcs can be when on the warpath. They initially blitz right over the humans — there's even a blink and you'll miss it shot of one orc doing a Neck Lift on a horse without any sign of strain.
Medivh singlehandedly turning that first fight against the orcs by using a spell to "ignite" the fel corruption inside the orcs is kind of awesome, but watching them scream and flail as they burn to ashes from the inside out is... unpleasant.
It's even more disconcerting when Gul'dan does the same before the film's final battle.