So I was thinking about the WOJ that Harry missed something when he was looking at the runes on the buildings on Demonreach, namely that they were similar to the Hag's ascension ritual in Welcome to the Jungle. So I reread that and noticed that Bob and Harry described it differently from the Darkhallow, as "makes you more of what you already are" rather than "poof, instant dark god".
Jim has ruled out Merlin being the English prisoner. So, what if he used an ascension ritual to turn himself into Alfred? Took the 'if you want it done right, do it yourself' approach rather than finding an existing genius loci to build around.