Genius loci

A genius loci is an elemental spirit connected to a landform crossed with ley lines.

Description
In classical Roman religion, a genius loci is the protective spirit of a place, and is often depicted in religious iconography as holding attributes such as a cornucopia, patera, or snake.

Demonreach
Demonreach's own genius loci is 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. Within the island's boundaries, it shows Intellectus, and is able to resist the power of Maeve.

Turn Coat
In Turn Coat, Harry Dresden uses a Sanctum invocation on it.

Cold Days
In Cold Days, it guides Harry Dresden and Bob below the surface of the island, showing them that Dresden's body passed the previous year in one of the chambers located there. Allowing Dresden and Bob to realize that below the island there is an enormous prison for powerful magical creatures, such as a number Naagloshii, dark gods and other nameless ones, Demonreach it informs them that Dresden is its warden.