how enchanted are we talking about.

Really? Nobody else picking army of demons? I can think of a whole lot of stuff to do with that.

I just want an endless maze that traps my soul for eternity, slowly transforming and corrupting me until nothing of my humanity is left and I am a horrible monster chasing down every new soul that enters the maze.

I'm stuck between the Enchanted Library and the Powerful Coven.

If the powerful coven means that I also have powerful coven powers, then powerful coven, but if not, enchanted library.

Do I get to pick the species and appearance of my terrifying familiar?

Definitely enchanted library, especially if I can shrink it and carry it in my pocket.