Modern Merlin shifting into place with all his memories and magic was beautifully conveyed, though I'm wondering why he feels the need to conceal himself from Arthur. In light of the fantastic things around them, I would think Arthur would welcome a weapon that can actually combat them. Morgana and Merlin touched on it, but I still wonder at what his reaction would be.

Merlin and Morgana's conversation was fascinating. Finally she's got one of her players back on her team. What a relief. You spun that window of relief on a dime with the picture you painted of those shambling corpses hunting our heroes. They have no idea what's going on above ground. It was already terrifying and that last line made it even worse.