I'm thinking he built Demonreach and put himself in there, for reasons having to do with back when Mab was mortal.
Seeing as how Mab is almost definitely Morgana.
Harry has ties to Merlin (the journals McCoy has, that sort of thing) but he isn't straight up him.
Yeah, I was being an idiot.
Im guessing in the apocalyptic trilogy that Demonreach is going to be cracked open, since thats what Justine seemed to be trying to do, so I guess well find out then
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I'm a model that is uncomplicated You can play a happy tune on me, but don't turn me off 'Cause then I am silenced