The Heresy is an enemy encountered in The Evil Within.
The Heresy is a large spider-like creature found in Chapter 12 "The Ride" while riding the bus. You may notice the creatures legs during the cinematic as the characters board the bus. It has two ways of attacking: raising its leg up and attempting to hit the player in a stab attack, or shaking its lower half and spawning maggot-like creatures which will grab onto Sebastian's leg and do very little damage. The maggots can be killed for ammo and the Heresy can be stopped from using its first attack by shooting it in the face with a sniper rifle. It is worth noting that the maggots may leave Sebastian open to a leg attack from Heresy
After a moment, the creature will raise its legs to crush the bus, but it is unintentionally killed when the bus goes through a tunnel. Upon killing it, the player will get the achievement Arachnophobia.
The creatures body is swollen with hideous lesions and boils, from which it will spawn the maggot creatures. The head appears almost humanoid, with swollen eyes and a nose, but large black pipes erupting from its mouth. Heresy does quite significant damage with the stab attack, so if Sebastian keeps moving you should avoid lethal damage. The head is a definite weak spot on the creature.