Grettir

Grettir was a dwarf Warlock.

Biography
He guarded the bridge leading the way to The Fisher King's kingdom, he referred to Prince Arthur Pendragon, Merlin and Gwaine as Courage, Magic and Strength respectively, as did the Fisher King himself. He met Arthur first when he crossed the bridge on his quest for the Trident, he noticed the Phoenix Eye on his wrist which Arthur said was from a friend, Grettir didn't warn him about it's life-draining effect. Grettir then met Merlin and Gwaine when they followed Arthur to save him from the Phoenix Eye.

Grettir was exceptionally skilled in the arts of Magic, he was able to turn Gwaine's sword into a flower when he threatened him and then back again. (The Eye of the Phoenix)

It has been confirmed that Grettir will return in Series 4.

Appearances
Series 3:

The Eye of the Phoenix (first appearance)

Series 4

Behind the Scenes
Grettir is played by Warwick Davis, who had notable roles in great films like, Star Wars and Harry Potter.

It is never mentioned that Grettir is a dwarf, we just refer that he is one base on his size (and he's in Merlin)