Witch Creek Road S4 E20 Commentary
SUMMARY: The Serpent lights a gas station on fire; the resulting smoke allows Tenebris to achieve his true form: that of a giant smoke demon.
TENEBRIS' TRUE FORM: The size and strength of Tenebris is directly related to the amount of smoke available to him. When we first met Tenebris, he was small and able to fit under a glass (due to having to rely on cigarette smoke). Now we're finally able to see Tenebris in all his awesome glory.
IN THE AIR: The Serpent mentions to Tenebris that he can feel it in the air, referring to the fact that something bad is coming. This is referring back to Garret's observation in Episode 19 that, "The air feels as though it lacks oxygen. Like he will, at any moment, start gasping for breath." This is also mentioned in the Grace section of Episode 19 with, "The air is bad."