When they burst into the Cortex, the first thing Barry notices is that the light isn't right. He can't quite put his finger on what's wrong with it, but he knows it isn't right.

The second thing he notices is that Cisco is collapsed on the floor in a heap, and when he rushes to his side he finally gets a good look at the prototype tachyon generator.

It is turned on.

The machine glows with a dark energy, and the air around it twists and tears, like a mirage but more angular – sharp in a way that feels like it physically hurts his eyes. And yet, it's hard to look away from the monstrosity, and the sucking void growing around it; something deeper than mere morbid curiosity has got its hooks into his psyche, pulling all his focus towards the rift and freezing him into place…

Gasping for breath, Barry manages to wrench his attention away from the anomaly and back to Cisco, pale beneath him. He shakes his shoulder vigorously, desperately – "C'mon Cisco, you have to get up! Get up!" – but he remains unresponsive; his pulse, when Barry seeks it out, is thready beneath his fingers.

"Try the salt!" Iris urges.

"If you wanted smelling salts you should have specified!" Feeling slightly hysterical, Barry complies, tossing a handful of ammonium chloride onto Cisco's chest. Nothing happens for a moment, and then the salt grains burst with brief light like a thousand tiny sodium flares, and Cisco begins to cough.

Groaning, he raises a hand to his temple. "Ugh, my head."

"Cisco! You're awake!" Barry exclaims.

The unexpected sound of slow clapping interrupts the moment. "Well Done. That Was Really Quite Clever." The voice comes from everywhere and nowhere, bouncing around the room without origin.

Barry turns in place. "Who's there? Show yourself!"

"Oh, You Wouldn't Want Me To Do That," The voice says. "Though I Daresay There Will Be Time Enough For That Later. Besides, You Already Know Who I Am." When it speaks, the low voice rasps like a shovel turning over grave dirt, setting Barry's teeth on edge.

Cisco struggles to his feet, leaning heavily on Iris and Barry's supporting arms. "You're… you're Dr. Wells… but… you're talking, how are you talking?"

"He's not." Iris steps forward. "It's not Harrison Wells. It's the demon that killed him. Eobardthawne. That's your name, right?"

"Wow. You Brought Yourself A Friend. Welcome, Miss West. Barry Always Said You Were A Smart One." The demon's voice is snide and mocking, and Barry has a hard time believing it's the same… person… that he's gotten to know over the past few months. "You're Just In Time For The Show."

The prototype pulses with an unholy light, and Barry quickly turns his head away from the nauseating sight. But, like a black hole, it seems to warp the fabric of reality; it is a place where parallel lines might meet, where perspective is next to useless, and it's leaking. Shadows fall upwards, dripping towards the suddenly high, cavernous ceiling.

"I've got a bad feeling about this," Cisco intones solemnly next to him, before he lists dangerously to the side; he would have fallen over if he weren't being propped up by Iris on his left side. His face is ashen as he pants for breath, and Barry's worry ratchets up yet another notch, even as white-hot fury blazes through him.

"What did you do to tachyon generator?!" Barry demands.

"Do? Nothing – It Is Operating Exactly As It Was Designed To. There Is Nothing I Could Have Done To It; I Have Been Without Tangible Form For Much Too Long. If I Could Have Done It Myself, It Would Not Have Taken Me Sixty Years To Reach This Point. You Cannot Possibly Conceive How Much I Loathed The Fact That I Had To Rely On Mortal Help. You Humans – As Pathetic And Contemptible As You Are Weak-Minded… Though You Do Have Your Uses."

"So, what are you saying? That it was all a set-up?" Cisco's hands clench into trembling fists. "Your grand plan to build a Doomsday device?"

"Well, It's Not Grand At All, It's Actually Very Simple. I Knew With Enough Modifications Even This World's Primitive Technology Could Be Used To Open A Gateway To My Home Dimension. All I Needed To Do Was Convince Someone To Build It For Me." The voice pauses for a moment. "The Fact That The Infernal Wormhole Will Unleash Hell Upon Earth Is… What You Might Consider An Unfortunate Side-Effect."

Barry can see, reflected in the glass of the chemical cabinet, a blurry, indistinct figure, with far too many limbs flickering in and out of existence - a sooty, sulfur-colored form with blazing red eyes, like pits of lava. It paces back and forth, and Barry's heart lurches into his throat. He turns to Iris, but she's still struggling with the sight of the tachyon prototype-slash-Hellmouth and doesn't seem to have noticed the figure yet. Next to him, Cisco sways unevenly, and his nose is bleeding, a thin trickle of blood dripping over his lips and falling to the floor.

Barry thinks he might now know where Dr. Wells – where Eobardthawne – is getting the energy to speak to them directly, and so for Cisco's sake they need to stop the madness and stop it quickly. It seems Cisco has just reached the same conclusion, because he wipes the blood from his face with a groan. "Aw man, tell me I'm not Ginny Weasley in this scenario."

"I'm So Sorry." The demon does not sound sorry at all. "I Have Grown Quite Fond Of You Cisco. In Many Ways, You Showed Me What It's Like To Be Human. I Did Not Anticipate, As Difficult These Past Sixty Years Have Been For Me, How Much I Would Come To Enjoy Working With You. With All Of You. And Yet. That Does Not Change What Needs To Happen."

The fiery eyes reflected in the glass grow closer and closer as the demon keeps talking… and then, the figure gets impossibly closer, as it moves out of the reflection and into the room – or else the room expands backwards into the mirror.

Barry has finally reached his tipping point – so overwhelmed by fear that it becomes a distant emotion, he has only one thought in his head.

Break the machine.

It's obvious that's what they need to do; if a machine is holding a portal open to the Netherworld, then they must first depower the machine.

Cisco would be devastated to lose his years of work on the tachyon prototype… but then again, it didn't exactly perform as advertised, did it?

Barry gently eases Cisco over so that Iris is bearing more of his weight. She glances over at Barry, and he tries to silently communicate his plan; if she can keep the demon monologuing, keep him distracted long enough, then Barry can try to slide closer to the lab bench, where a very nice wrench might be just the thing to throw at the prototype's power cable.

Fortunately, Iris gets the message, and she turns to face the demon more fully. Barry is impressed with the way her voice barely shakes, despite the tremors he can see in her hands. "You killed the real Harrison Wells. I want to know why."

"You Might Think It Was Out Of Spite For Daring To Summon Me, Trapping Me In This Pitiful Plane Of Existence, But From My Perspective, He Was Less Than An Ant Beneath My Foot. Your Mortal Lives Are All So Brief, So Spectacularly Uneventful And Less Than A Fraction Of A Blink In The Face Of Eternity; What Does It Matter When You Die? What's Thirty Years, More Or Less?"

"That's monstrous."

"I Know You See Me As The Villain. But If You Were To Look Back, Look Back Carefully At Everything That I Have Done, Every Wheel That I Have Set In Motion, You Would Realize I Have Only Done What I Had To Do. Nothing More, Nothing Less."

"Did you kill Hartley's rats?" Cisco's voice is slightly slurred, and his breathing is heavy. "What purpose did that serve?"

"Oh. Well. I Got Bored."

Reaching behind him blindly, Barry's fingers finally brush against cool metal, and he lifts it carefully with a sweaty grip.

Across the room, the yellow demon's many insubstantial limbs suddenly blur, and a funnel of wind picks Barry up and slams him into the wall. The demon traverses the length of the lab in an instant and stands in front of him, less than two feet away. Even this close its features are hard to make out – something in his brain rebelling against the thought of resolving the monster in greater clarity, maybe. The air thrums around it, and Barry feels like a plucked piano wire, stretched taut and vibrating at whatever hellish frequency the demon exudes.

"You Can't Stop Me, And You Never Will." Its breath in Barry's face is hot and rank, like meat left out in a fetid swamp for too long. Barry's throat is tight and he can't get enough air; he thinks the demon might be choking him, but all he can see are the red, red unblinking eyes.

"With Barry and Cisco in this fateful hour," Iris cries out from somewhere to his right, "I call on Heaven and all its Power…" The demon releases Barry and turns towards Iris; Barry whacks it as hard as he can with the wrench still in his grip, but Eobardthawne does not notice.

Iris continues her chant, steadily gaining volume. "The sun with its brightness; and the snow with its whiteness… and, and the fire with all the strength it hath - " With each word the demon seems to falter, its form condensing further and further until it almost resembles a man. A low, wordless, continuous snarl vibrates from the beast, and it lunges for Iris – but Cisco has already made a circle of salt around them and Eobardthawne is forced to stop, however momentarily.

"- And the lightning with its rapid wrath; and the sea with its deepness, And the rocks with their steepness; all these I place, between myself and the powers of darkness!"

At the final word, the demon shrieks, a terrible, dread sound that makes all the hairs on Barry's arms stand on-end, as he covers his ears in desperation. Eobardthawne continues to cry and thrash, even as the demon breaks apart and comes undone before their very eyes, dissolving into nothing until not even ash remains.