Nyarlanthotep's boss theme is Unworthy.
Nyarlanthotep appears as a brown tentacled monster with various green eyes over his body and a gaping maw.
After the player defeats Jakob in a duel, a stairway to the level's exit will open. Just before reaching sunlight, however, the entire room will fade to black. Nyarlathotep then proceeds to make a speech to the player:
"He was worthy, you know. And he accomplished so much... but then... you came along. You who were stronger. You who were faster. You... I like... better. Who did you expect at the end of all this? God? The Devil, perhaps? No... it's just... me."
The player will find themselves in Nyarlathotep's arena, where he makes his grand appearance and reveals that he is the source of the Cult's power. From this point, the boss battle begins.
Nyarlathotep has 17500 health, giving him the second highest health pool in the game. His main attacks consist of swiping at the player or slamming down onto the platform the player is standing on. Nyarlathotep is at first invincible, however shooting cracks in the ceiling will weaken him with light, opening his chest cavity. The player will need to accomplish this about four times in order to successfully take him down.
Nyarlathotep is weak to stronger weapons like the Riveter, Hunting Rifle, and Mortar. However, due to the range between the arena and him, close range weapons like the Shotguns and Super Shotgun are less effective against him. The best strategy to avoid being hit by Nyarlathotep is to watch his arms and determine which one will be used to attack first. Lower arms will be used for swipe attacks, while the upper arms will be used for slam attacks.
Upon successfully bringing Nyarlathotep down to 1 health, he will become invincible for the final time and deem that the player is worthy. He grants some of his power onto the protagonist, corrupting them with the same energy used by the Cult and the Crystal of Madness.
- "He was worthy, you know. And he accomplished so much... but then... you came along. You who were stronger. You who were faster. You... I like... better. Who did you expect at the end of all this? God? The Devil, perhaps? No... it's just... me."
Upon being struck by light:
- "Ah! Cursed light!"
- "Curse you!"
- "Uuugh! You fool!"
After the battle:
- "Enough! You are indeed worthy... and so now... my power... is yours!"
- "How does it feel? Horrible, isn't it? But, wonderful, yes? laughter Who you were before is insignificant, who you are now is inconceivable. But here, you shall remain, 'till I have need of you again... for one must always reign... beneath... DUSK. maniacal laughter
- "Pray, that you never meet me in my thousand other forms."
- Nyarlathotep is an outer god featured in the works of H.P. Lovecraft. Nyarlathotep is also the only outer god to be able to interact with humans directly.
- Nyarlathotep's inclusion as a final boss seems to be a nod to the first Quake game, where the player fights Shub-Niggurath at the end, another deity created by H.P. Lovecraft.
- Nyarlathotep is voiced by Stephan Weyte, the voice of Caleb from Monolith Productions' Blood series, one of DUSK's inspirations.
- Before Nyarlathotep appears, several references to him, and the Lovecraft story of the same name hinted at his eventual appearance.
- E2M8: Blood & Bone has a blood scribble: "The Darkness is Spreading" before the locked Blue-keyed door, the adjacent room could be described as a "choking room" due to the pitch darkness, stairs, and floating red dust.
- E3M2: Fire and Ice contains two direct quotes from the poem. "The Waves of Destruction", and "The Dimming, Cooling Sun".
- E3M4: Crypt of the Flesh indirectly mentions Nyarlathotep in graffiti. "The Chaos Crawls To Me"
- E3M9: The Dweller in Darkness - one of August Derleth's stories about Nyarlathotep was entitled "The Dweller in Darkness".
- E3M5: Blasphemy reveals Nyarlathotep as the tornado that destroys the temple, leading to what is beneath the altar.