This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The 3rd Birthday
So, you have to fight a giantEldritch Abomination inside... anotherEldritch Abomination, while floating around in what seems to be rather beautiful sphere. You shoot at it... a LOT, and this thing slowly approaches you without twitching. When near you, it paralyzes you with its Frickin' Laser Beams, catches you with its tentacles and eats you whole. Your only hope of survival is to transfer to another body. From there the whole pattern begins anew, all while you have to witness how the monster eats your former body, gallons of blood and the sound of bones being crunched included. Well, that'sThe 3rd Birthday. Oh, the horror.
It doesn't help either that Aya's expression at the sight of this thing is blank horror...
From the soldiers' perspective, the fight with Hyde could easily qualify as this. One of the soldiers randomly walks up to him and Hyde starts talking to him for no visible reason. The soldier suddenly raises his gun to shoot, and HydeArmor Piercing Slaps him to the ground. Right afterwards, he transforms into some kind of half-Twisted monster with disproportionate limbs and tentacles sprouting out of its back. I'd be freaked out...
A bit of Fridge Horror here: I'm sure many of us have dived out of a dying soldier - and heard the screams - but what do you think the soldier him/herself went through? If they are conscious when Aya possesses them, that's kinda creepy, but it's worse if they're not. One moment you're in cover, a fellow soldier stumbles for a moment, the next you're squirming on the ground with blood pouring out of every orifice.
The horrific bloodbath that ensues when the Twisted invade the concert in the game's second mission.