Agents of SHIELD has been a bit on even during all those scenes when Robbie Reyes' Ghost Rider and his art deco flaming skull haven't been on screen. But now things could pick up, as it finally revealed how it's going to brush up with the upcoming Doctor Strange movie — courtesy of an ancient, evil book called the Darkhold.
The Darkhold was briefly mentioned back in episode two, but the reference became a lot more overt when Robbie Reyes' uncle Eli explained how the book helped his employer Momentum Labs make a quantum particle generator, which blew up, and turned its researcher into the vengeful and very silly-looking ghosts that have been moping around AoS this season. Coulson refers to it as "a book of sins, a book of spells, [with] infinite knowledge," which is presumably why an ancient book has instructions on how to assemble quantum particle generator.
The Darkhold is of course part of the Marvel comics universe, with a fascinating history that does indeed link it to Doctor Strange. The book is one of the oldest tomes in the world of Marvel mysticism — written by an Elder God named Chthon way back in the past, as a repository of literally every evil spell he knew. In terms of the comics, it first appeared in 1972, in Marvel Spotlight #4, and a copy of it eventually found its way into the hands of Doctor Strange, who's had to call on its dark powers several times over the years.
Surprisingly Doctor Strange isn't the Marvel character with the most history with the book. As it turns out, Chthon originally wrote down the Darkhold as a series of scrolls, each containing powerful dark magics — scrolls which played major roles in Marvel's Conan the Barbarian and Red Sonja series in the '70s, as well as several other of Marvel's horror comics. They were eventually were retconned into a single grimoire — conveniently with a big letter "D" written on the front, collected from Chthon's scribblings by Morgan Le Fay, a sinister sorceress from King Arthur's era who has fought Strange, the Avengers, Spider-Woman, and more.
Perhaps most importantly, the Darkhold is responsible for the creation of vampires in the Marvel universe. Spells from it were used to raise Varnae (who was a member of the Darkholder Cult, a society dedicated to using the Darkhold for nefarious purposes), the first vampire, from the dead, whom Conan and Red Sonja battled. The Darkhold also managed to create werewolves in the Marvel comics-verse. At one point, the book even got its own ongoing comic in the '90s, Darkhold: Pages from the Book of Sin, which saw a demonic entity named Dwarf sell pages of it to desperate members of the public, at the cost of their souls.
While it currently resides in Strange's collection of magical books — in the comics, who knows where it currently resides in the MCU — the Darkhold has been used by other Marvel characters over the years, like Blade and Doctor Doom, searching for power magic and cures for supernatural afflictions.
It's one of the most powerful items in the Marvel universe, and it's cool that it's made its way to Agents of SHIELD, although it loses a bit of its sinister power when the only villain chasing after it seems to be a grumpy ghost. But at least this opens the door for Conan the Barbarian and Red Sonja to join the Marvel Cinematic Universe. Of course this will never ever ever happen, but... it technically could, and that's enough to make me happy.