Scarlet Witch, and she explained "there’s an entire chapter devoted to you in the Darkhold."
In the comics, the Darkhold is one of the most powerful and dangerous books of black magic there is. It was created millennia ago by the Elder God Chthon before he was banished from this plane of reality, and it has a baleful, corrupting influence that allows Chthon to reach into the world through every spell contained within its pages. The Darkhold is the source of some of the greatest evil supernatural beings; a spell from the Darkhold created the first vampire, and it has been tied to the creation of werewolves as well. WandaVision ended with Scarlet Witch unwisely reading from the Darkhold and learning magic from it, no doubt unaware of the terrible risk she is taking. The Book of Cagliostro in Doctor Strange put the warnings after the spells, meaning Stephen Strange had no idea what damage the spells he was learning could do, but the Darkhold wouldn't include any warnings at all.
Elizabeth Olsen's Scarlet Witch is due to return in Doctor Strange 2, and it's reasonable to assume Chthon will be the villain (not least because WandaVision contained several other nods to Chthon as well, notably including his Chaos Magic). That means Marvel is drawing inspiration from some of their darkest supernatural stories, fitting with what Olsen has described as a "horror show vibe." But it also means Doctor Strange 2 could introduce two new magical groups into the MCU - one an ally of the Masters of the Mystic Arts, the other an enemy.
The Darkholders
First up there are the Darkholders. In the comics, this ancient cult is devoted to the worship of Chthon, and they predate the fall of Atlantis itself. A group of proto-Darkholders began the process of collecting the scrolls Chthon had inscribed his original spells upon; they wrote them down on parchment, which they bound together into the book called the Darkhold. Naturally, exposure to Chthon's words corrupted them, and these nascent Darkholders became sworn servants of the Elder God. They were responsible for circulating rumors that the Darkhold could be used to secure power and immortality, leading many to covet its magic.
While some of the Darkholders are human cultists, many are mystical entities in their own right, who have been transformed by the dark magic they peddle in. There are vampires and even demons among the Darkholders, and their most prominent figure - who appears to serve as a leader, a direct conduit with Chthon's will - is a being called the Darkhold Dwarf. "I've had many names across many ages," the Darkhold Dwarf observed on one occasion. "So many even I forget a few. For ease of reference, call me what most people I meet do: The Darkhold Dwarf. Insensitive, I it, but at my age, frankly, I don't give a damn." He is best known for ensuring pages of the Darkhold get to the place where they will cause the maximum disruption, and his overarching goal appears simple; to bring Chthon back to Earth. Although the Darkhold Dwarf prefers simply to meddle, he possesses tremendous magical power, sufficient to repel a magically-boosted Carnage with astounding ease.
It wouldn't be difficult for Marvel to retcon the power-hungry Agatha Harkness as a Darkholder, but if she is not then cultists - servants of Chthon - could be circling to make sure the Scarlet Witch serves their purpose. The unrestrained magic of the Darkhold serves as a beacon to Darkholders, meaning they could well be drawn to Wanda as she learns from this forbidden tome. In the comics, to read the Darkhold is to fall increasingly under Chthon's influence, and they would naturally be delighted to see Wanda turning its pages - although they likely wouldn't make their presence known unless they felt she needed a helpful nudge of some sort.
The Children of Midnight
The second group are less well-known, but just as intriguing. In the comics, the Midnight Sons are traditionally a group of superheroes who gather together to oppose the forces of supernatural evil. While many fans would no doubt be delighted to see the Midnight Sons in the MCU, it's probably too soon, simply because they include too many powerful magical heroes who have yet to be introduced in the movies, such as Blade and Ghost Rider. That doesn't mean the concept couldn't be used, though - and Gerry Conway's Carnage run may offer a clue as to how. That series reinvented the Midnight Sons as the "Children of Midnight," an ancient order who had been formed to protect the Earth from Chthon.
The traditional leader of the Children of Midnight is a member of the Montesi family. Centuries ago, the Pope entrusted care of the Darkhold to the priest Giacomo Montesi, who possessed an unusual resistance to the Darkhold's corruptive influence. Giacomo received a special dispensation from the Pope to marry and have children, in order to establish an entire bloodline who shared his resistance and could serve as the first line of defense against Chthon. The modern leader of the Children of Midnight is Victoria Montesi, a woman who conceals a dark secret; her father Vittorio was sterile, and in a moment of weakness he used a fertility spell from the Darkhold. Victoria was the key to Chthon's plan, for he impregnated her with his own essence; fortunately Doctor Strange managed to intervene and avert the end of the world. She subsequently dedicated herself to the continuation of the Montesi mission, using her own connection with the Darkhold to spot wherever its magic was being used.
The Children of Midnight are a little harder to introduce into the MCU, simply because you'd expect them to have been drawn to the Scarlet Witch simply because of her Chaos Magic, which is also derived from Chthon in the comics. Still, it is worth ing that - just like SWORD - the Children of Midnight would themselves have been thrown into chaos by the Blip, with half their number erased - perhaps even including those crucial Montesis. Chthon could well have timed the creation of his Scarlet Witch well, in order to capitalize on the Blip. If so, Doctor Strange 2 could see some of the Children of Midnight step out of the shadows and ally with the Masters of the Mystic Arts.
