Goblin's Gold

Tropes

Bluff the Imposter: Arthur does this with the possessed Gaius, only to be immediately knocked out afterwards.

Cringe Comedy: The goblin covers Sir Leon in boils, gives Arthur donkey ears, makes Uther's hair fall out, and forces Morgana and Gwen to break wind in front of the entire court.
Curiosity Is a Crapshoot: Should have just left that box alone, Merlin.
Curiosity Killed the Cast: A non-fatal example, when Merlin releases the goblin.
Demonic Possession: The goblin has the ability to possess people.
Fantastic Arousal: Arthur's reaction when Guinevere begins to stroke his donkey ears.

For the Lulz: The goblin's only motivation (outside of accumulating gold).
Let Us Never Speak of This Again: Arthur suggests this to Gwen. She's very receptive to the idea.
Our Goblins Are Different: Small and vindictive.
Screw Politeness, I'm a Senior!: The Goblin-in-Gaius gets away with a fair bit of bad behaviour thanks to this trope.
Sealed Evil in a Can: The goblin, who was sealed in a box, then escaped to torment everyone in the castle, and then resealed after being trapped by Merlin and Gwen.

Tasty Gold: A variation. Goblin-in-Gaius doesn't lick gold to check its authenticity, but because it tastes good.
Tattooed Crook: The goblin has multiple tribal markings or tribal tattoos of what appears to be the sun.
The Trickster: The goblin.
Unusual Ears: Arthur temporarily gets donkey ears.
What Happened to the Mouse?: Merlin finds the trapped goblin in a secret room behind a bookcase in the library. It's filled with magical books and paraphenalia, but he never revisits it and we never learn anything more about it.