User blog comment:Fimber/The "Merlin" finale ("The Diamond of the Day")/@comment-60.242.64.101-20121224033541/@comment-5102537-20121224094823

Right, I've wondered about that too. Maybe the one who put the box under Merlin's bed was a sorcerer? I don't know but it would have been nice to get an idea.

As for Gaius, I'm not sure if the slug takes away only magical powers that people are born with or also the ability to practise magic due to studies. Gaius wasn't born with magic but only learnt how to use it. Maybe that's the reason?