Demonreach

Demonreach is the name Harry Dresden gave to an island in Lake Michigan, which Karrin Murphy refers to as Hard Rock. The island first appears in Small Favor.

Description
Demonreach is an island in Lake Michigan that was removed from maps after a long association with the supernatural. The energies around the lake deter travelers from approaching and slowly become hostile to those who approach.

According to the backstory in Cold Days, the island's magic was created and put in place by the Original Merlin across time as a supernatural prison to keep the worst and most powerful evil beings contained. There are at least half a dozen naagloshii imprisoned there; these are considered the most benign, low-security inmates. Other entities, described as "[n]ightmares, dark gods, nameless things, immortals" are also imprisoned there.

The island was once the location of a cannery in the early 1900s, until the magic from what Harry Dresden believed to be a ley line running through the island killed enough of its inhabitants to chase them away. It is later revealed that the ley line does not run through the island, but from it; the island, particularly the supernatural prison far below it, is the source of the ley line's tremendous power. The Gatekeeper Rashid has advised Dresden not to use that power directly, only the Intellectus of the island's spirit.

Dresden is currently the Warden of this prison, a duty he inadvertently took on when he performed the Sanctum invocation and bound himself to the island.

Dresden describes it as covered in woods, lots of trees under a foot in thickness. Between the trees there is brush and thickets with thorns growing about five feet up from the ground. There is an old ghost town along the shoreline, so old that trees have reclaimed the space and most of the buildings have fallen down. There are the rotting remains of an old dock with only the posts sticking out the water. Up on the highest point of the island on a hill stands the outline of a worn building or tower.

In Turn Coat, Harry Dresden uses the Sanctum invocation on the island and its genius loci. He names it Demonreach. With Thomas' help, he constructs a floating dock, which they name "Whatsup Dock".

The island is known at some point to have had some sort of conflict with Senior Council member and Gatekeeper Rashid.

The stones on the cottage and on the tower glow with fox fire, then evolve into symbols, script in some unknown language. They seem to repulse the naagloshii, keeping him from approaching the structures. Bob tells Dresden that he doesn't know what the symbols on the rocks of the cottage are except that they predate wizardry and are very powerful and very numerous—hundreds on each rock—and layered.

In Cold Days, Dresden stated that the spirit entity of Demonreach spoke to him at Graceland cemetery, the year before.

Genius loci
The island's genius loci is extremely old and powerful. It has Intellectus limited only to its shores, which Harry Dresden can use while on the island. Demonreach's genius loci manifests as a figure about eleven or twelve feet tall, with a billowing black cloak, a limp (caused by the glacier that carved out the lake), and two vividly green eyes.

In Cold Days, the manifestation of the island manages to resist the power of Maeve, the Winter Lady (one of the most powerful Sidhe), and a retinue of high ranking Sidhe Nobles, while simultaneously warding off an attack from an army of outsiders. It took the combined powers of both Ladies and their retinues hours to even damage him slightly, a testament to his immense power.

However, Demonreach seems limited in its ability to actively resist invaders, which Dresden speculates is due to the nature of the island itself. Mab acknowledges that Demonreach could have responded more aggressively to her daughters' attack and thanks him for being restrained.

Small Favor
In Small Favor, the Denarians took Marcone and the Archive hostage and brought them to the island. Dresden, Michael, and Sanya go there with an agreement to trade eleven Coins and Fidelacchius for Ivy. The island feels familiar to Dresden, though he's never been there before. The trade turns into a rescue and chaos breaks out.

Turn Coat
In Turn Coat, Harry Dresden uses the Sanctum invocation on the island and its genius loci. He names it Demonreach.

Morgan is falsely accused of murder, and Dresden invites all the parties of interest to witness his solution to the mystery: Shagnasty, Lara Raith, the White Council... and hopefully the traitor.

Ghost Story
In Ghost Story, Harry Dresden's body is kept alive with the help of Queen Mab's power, Demonreach embedded vines into his body to feed him nutrients and the spirit entity growing in his brain who kept his blood flowing.

The statue of Eternal Silence that speaks to Dresden in Graceland cemetery is probably Demonreach. Dresden recognizes it later on, in Cold Days.

Cold Days
In Cold Days, Harry Dresden and Bob investigate the information provided by the Leanansidhe that the island will blow up in the following day or two, destroying a large chunk of the Midwest. In Demonreach's cottage, Bob admits that the magic he encounters is way above what he knows or what Dresden and all humanity can perform. The genius loci Demonreach guides the two of them underground, showing it as an enormous prison for magically powerful creatures, such as Naagloshii, dark gods and other ones. Dresdn's body spent the previous year there, and the genius loci informs him that he's the warden of the place, with its responsabilities.

Bob translates for Demonreach by creating a movie of how The Original Merlin constructed the prison by repeating the spell at the same place at different times but at the same time.

Dresden hosts a parasite in his head causing him headaches; it helped keep him alive while his spirit wandered. Its price was that Dresden not be told any details about it. Eventually it will burst out of his skull and kill him.

Maeve and Lily inform Dresden that Mab has gone mad and plans to unleash chaos on the world that night—Halloween. Dresden and his team try to determine who is casting the spell required to release the monsters, and should they be Outsiders, who let them into this world; while they can't lie, Maeve and Lily could be wrong.

Dresden leads the Wild Hunt, including the Erlking and Kringle, into battle with Outsiders on barges and numerous other boats led by He Who Walks Before along the shoreline of the island. They advance up the hill, until a strong Circle blocks them with a wall of deadly power. Being part of the Island's architecture, only Dresden manages to pass through. Inside, he finds Maeve and Lily trying to destroy Demonreach's spirit entity using the island's Circle of Power. Fix engages Dresden in battle, but is easily defeated by Dresden's use of Intellectus. They join forces to stop Maeve.

Skin Game
In Skin Game, Demonreach's spirit helps Harry Dresden in keeping his migraines under control, something increasingly difficult with the passing of time, and is jokingly nicknamed Alfred, after Batman's Alfred Pennyworth. The spirit appears unimpressed, but Dresden officially dubs it Alfred Demonreach.

Dresden also notes that he's much more familiar now with the Island's defenses and how to use them, and he would have no trouble defeating even Sharkface in a rematch, if it were held on the Island. Dresden becomes so comfortable on the Island that he forgets how spooky and hostile it normally seems to others, and wonders why his friends don't come visit often.

Word of Jim
Per Jim Butcher, “(Rashid) has been on the island a couple of times, and it’s never gone well.”