Demonreach

Demonreach is the name Harry Dresden has given to an island in Lake Michigan. As of Turn Coat, Dresden has claimed it as his Sanctum.

History
The island was once the location of a mining town in the early 1900s, until the magic from what Harry believed to be a a ley line running through the island killed enough of its inhabitants to chase them away. It was later revealed that the ley line does not run through the island, it originates at the island where it wells up from some unspecified location. Today, the island itself is uncharted, located somewhere in the coral-infested waters of Lake Michigan.

The island is known at some point to have had some sort of conflict with Senior Council member and "Gatekeeper" Rashid. Per Jim Butcher, "(Rashid) has been on the island a couple of times, and it’s never gone well"

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

After discovering Harry's new connection to the island, the Gatekeeper Rashid advised him not to use the power of the island, only the awareness it supplied. The island presumably has a very dark nature that Rashid fears will affect Harry somehow.

In Ghost Story, Harry Dresden's body was kept alive through a mixture of Queen Mab's power and Demonreach embedding vines into Harry's body to feed him nutrients until he could return to life and resume his position as the Winter Knight.

Genius loci
The island's genius loci is an extremely old and powerful one. It has Intellectus confined to its shores, which Harry Dresden is able to use while on the island. Demonreach's genius loci manifested as a tall figure with a billowing black cloak, a limp (caused by the glacier that carved out the lake ) and two vividly green eyes during its duel with Harry Dresden.