In Iceland, they call it the Black School. (But blá can also mean “blue.”)
There go all the aspiring young warlocks to learn from the Horned his secret and magical arts.
Exactly where the School may be is hard to say. (Some say Paris.) It's one of those places that seems always to be somewhere else.
It's called the Black School because it's always black as night there. (It sounds like some sort of cave.) For five (or nine, or seven) years, they live there together, underground, in the dark.
There they study from the Horned's ancient tomes, which, being written in letters of fire, can thus be read in the dark.
Each day they receive for their sustenance a trencher and horn from the hand of the Horned, although they do not see him.
And of the Black School there is also this to say: that at the end of their study, when they step out into sunlight for the first time in five (or nine, or seven) years, each warlock must leave behind his shadow there with the Horned, and so casts no shadow for all the rest of his days.