It's all a Nemesis plot.

One of the main points of PT/BG was the desire by Nemesis to hasten the Apocalypse. Go back and reread the conversation between Harry and Justine. It explains Thomas's attack on Etri and how the whole Ethniu/Formor attack on Chicago was a diversion for another attack on Demonreach. Basically, Nemesis monologues to Harry that it has been behind almost everything he's encountered and that it is the overarching villain of the piece.

Yes, there are individuals such as Cowl, Nic and Drakul running about. But almost all the other foes that Dresden has faced have been pieces in the game Nemesis is playing against the protectors of reality in this universe and even they are not entirely clear of his influence.

The Outsiders abhor creation and especially humans, they desire complete destruction. Everyone else wants a new creation along their own lines, but they are all quite happy to see the end of humanity.

Cowl quite clearly has the ear of the Outsiders (or some other equivalent sensory appendage, I hope, rather than some other sort of appendage), and as such is their man in creation, but he is likely playing his own game,  Nick is Lucifers guy, Drakul’s his own guy, Harry is Uriel’s guy.

We'll have to agree to disagree.

The BC, and I include Nemesis in that, wants dissension,  I agree, but the Accorded Nations are tighter together after fighting together,  that would have happened whether they won the battle or lost it.  Drakul, I don't see him working for anyone, he was just on a recruiting trip.

For now, I'll stand on my belief that Etri's attempted assassination and Ethniu's attack were separate plots by separate groups.   But others used it to pursue their own goals.

Unless or until we get additional evidence one way or the other.

Thomas, struggling to speak.
 I thought of my brother’s face, crushed and swollen out of shape.
 Junghg. S’Jnngh.
 He hadn’t been able to say, “Justine.”
 Or maybe he hadn’t been trying to say it.
 I thought of the island, disturbed at the great powers expended that night.
 The last thing I needed was something slipping out of the prison during all the hubbub.
 S’Jnngh, he’d told me.
 Why had my brother gone after Etri?
 S’Justine, he’d told me.
 It’s Justine.
 Hell’s bells.
 It was Justine.

“See, there were just too many threads being pulled,” I said. “The attack on the Outer Gates especially. And the Titan herself . . . God, what a blunt instrument. What a big, loud distraction. So that you could get inside.”
 Justine’s head turned to face me. The lightening sky was behind her. There was nothing to be seen of her expression but blackness.
 I limped forward a couple of paces. Nothing specific was any worse than it had been an hour ago, but even the immunity of the Winter mantle had its limits. My joints felt like they’d been dipped in plaster and were slowly drying stiff.
 “And every single living family member of mine, personally, was placed in danger. All of them. To make sure I had the maximum amount of personal worry to distract me.”
 Justine has incredible cheekbones. They shifted, slightly altering her shadowy profile as she smiled.
 “Something about Justine wasn’t . . . quite right, earlier, in the apartment,” I said. And I let my voice harden. “How long ago did you possess the girl?”
 Justine was silent for a moment. Then she shook her head and said, “I think the problem is, you just don’t sound all that bright, wizard. Perhaps it’s skewing my expectations.”
 She turned toward me, slim and graceful, steady on the deck.
 I faced her and tried not to pitch over the rail as the Water Beetle bumped along the waves. It had been a long night. And I didn’t have much left, physically or otherwise.
 “Tell me your name,” I said, and slid some of my will into my voice.
 “You know who I am,” Justine purred in answer.
 Then she reached out with one hand and ripped a four-foot section of the ship’s steel handrail off its metal struts.
 I blinked wearily and fancied I could hear grains of sand pattering to the deck from my eyes. Now I knew what Ethniu had felt like at the end. “Humor me,” I said, with more of my will. “Tell me your name.”
 Justine, or whatever being was driving Justine’s body around, turned toward me and began slow, stalking paces forward. It made some abortive, choking noises in its throat, and then said, the words drawn from it reluctantly, “It will do you no good once I’ve caved in your skull. Nemesis am I called.”
 There. Bingo.
 For years, shadowy forces had been driving events in Chicago and in the wider world. For years, I’d been picking up threads and finding them connected to others. For years, I’d been flailing around trying to get an idea of the forces that had been arrayed against me.
 And tonight, one of the players was in the open.
 Right there. Behind Justine’s eyes.
 And I was going to get answers.
 I didn’t have much left in me but pure, stiff-necked, muleheaded contrariness.
 But even after the night I’d had, I still had plenty of it.
 “I don’t care what they call you,” I spat. The effort of maintaining my will made it impossible to move my feet as the slender girl stalked forward with her steel bar. “Thrice I say and done. Tell me your name.”
 The slender figure froze in front of me, shuddering.
 Then she exhaled in a slow, utterly sensual voice, “I am the doubt that wards away sleep. I am the flaw that corrupts, the infected wound, the false fork in the trail. I am the gnawer, the worm in the book, the maggot that burrows in the mind’s eye.”
 She shuddered in bizarre ecstasy and panted, in a frantic whisper, “I am He Who Walks Beside.”
 Hell’s bells.
 A Walker.
 And if I hadn’t twigged to its presence, I would have set it loose on Demonreach— the prison for the great nightmares of the world. Ethniu wasn’t the biggest thing in it—not by a long shot. And an Outsider with the power of a Walker, turned loose inside the island’s defenses, might well be able to destroy them and set loose every horror inside.
 Hell. There’d have been an Ethniu for every city, if the place got emptied out.
 The weight of my will, once finished forcing the information from the possessing being, flooded out of me and left me barely able to stand. I staggered back, away from the slender figure in front of me.
 Justine, calmly, pursued.
 “I hope it felt good to scratch that itch,” she purred. “This is the end of your story, starborn.”
 “How long?” I asked. “How long have you been in Justine?”
 Justine waved the steel bar in a vague gesture. “Mortal time is such a limited concept. A few years. Ever since she became close to Lara.”
 I glowered at her. “You conceived my brother’s child intentionally.”
 “Obviously,” Justine purred. “That ridiculous instinct, honestly. It is your kind’s greatest weakness. Once he understood that his mate and his offspring would die if he did not follow my instructions, well . . .” She shrugged.
 “So you sent him at Etri. At the svartalves, someone almost everyone respects. Why? To shatter the Accords?”
 “Apocalypse isn’t an event,” Nemesis murmured. “It is a frame of mind.”
 I probably would have staggered anyway, but the phrase hit hard.
 “This was less a plan than . . . an act of faith, I suppose you would say,” the Outsider continued through Justine’s lips.
 “Faith?” I asked.
 “In what is coming,” the Walker said. “The unraveling of all things into darkness and silence.”
 “Empty Night,” I breathed.
“Empty Night,” the creature echoed, in the hushed tone of a holy phrase. “So we pressed the attacks at the Outer Gates. While I sowed havoc within the walls of reality. We loosed some of the primal forces of your own precious Creation against you. Undermined Mab, her people, the Accords, the delusion of order you force upon the universe with your useless presence.” She smiled, dropping lower, the motion feline, sensual, hypnotic. “You may have survived the day. But the deed is done. We are the tide. Infinite. Unrelenting. And one day, starborn, make no mistake, we will wipe away all that you know. All we need is a single opening.”

  Let's not forget that Darkhallow is one of the things that runs through desperate Harry's mind along with trying to call back Lasciel's coin as worst, worser, and least worse option of becoming Winter Knight as he laid on his broken back in his office in Changes, to save little Maggie.

I think that The Darkhallow strength is dependent upon the circumstances, Halloween is a peak time to perform it where you can draw in the most power. Do it any other time it will give you a power up but not to God level, more in the Denarian/Winter Knight level. There may be other factors which affect its power, such as the presence of The Erl King in the mortal world.

Kemmler might have already done such a ‘weak’ Darkhallow, he was so much stronger than the average wizard, but unable to achieve the circumstances for a full Darkhallow during his lifetimes. The Erl King was in the 19th C Wildfae, by Dresden’s time he’s sworn fealty to Mab. This may have been to protect him from being summoned by Kemmler and Nameless - certainly when the latter was paperclipped into Winter.

It’s Checkov’s Darkhallow

It might require a Starborn for peak effect, which is why Drakul wanted it BUT Harry would only use it if it didn’t harm innocents. Ripping the Lifeforce from say a Fomor Army in achieving it would help ESPECIALLY if he has the Blackstaff to shield from the harmful effects and the Spear to maximise success. We have literally seen Eb rip the life from a hundred Red Court flunkies in the same way the Darkhallow would.

Any innocents could hide behind the Placard THAT would shield them from the Darkhallow as would the Carpenters House.