Most of Call Of Cthulhu: Dark Corners Of The Earth.
Some of the bosses are SO F**KED UP THAT YOUR CHARACTER WILL LITERALLY GO INSANE IF YOU LOOK AT THEM TOO LONG.
That, or Drakenguard 2 when you break the seal that kept the Gods out of the world. The first time I saw them and very shortly into the next level my only thought was, "JESUS CHRIST, WHY THE F**K DID YOU RELEASE THE SEALS?"
Oh, also shortly before you break the seals, you fight a little girl in a shadow dimension where there are about 50 of her with glowing red eyes, torn dresses, and a bloodstained butcher knife with teddybears and stars drawn on all of the walls with blood. It was like the worst Tonberry fight ever.