This ring can be found in the campaign Under the Burning Suns, in the 4th scenario Descending into Darkness.
This ring will increase your maximum speed by 1.
"How can he move that fast? He is faster than any goblin rider I have ever seen. It’s almost unnatural. [...] Wait a minute. He was wearing a silver ring on one of his fingers. I think the ring might be magical. Maybe that’s why he was moving so fast. ~Kaleh~