Looks like I've found the entrance to the "sunken citadel" Keeper Rafe talked about in his journal. According to him, the Glyph Key disappeared underground some time too long ago to think about. Rafe also mentioned caverns "swarming with strange creatures" – guess that's why the Keepers put a glyph lock on the entrance. It would be nice to know what sort of creatures, but the Keepers have never been much for useful detail. Turns out, not only did the Glyph Key disappear down here, but so did Rafe. The Keepers never sent anyone after him – but I don't expect he's still exploring the place. I don't have any good leads on where to look for the Key, but I can start by looking for Rafe's remains.