Remnant is full of legends. Some true, others not so much. Trouble is I know a lot of those legends. I don't know how or why I know, but I do because the other me knows them. Trouble is the me here doesn't know which ones are true and which ones aren't.
There is a legend about people born with silver eyes like Ruby's. A long time ago way back before huntsmen and kingdoms it was said that those born with silver eyes had the power to strike down a Grimm with a single look. I don't know how they were able to do it, but I think that's what happened to the dragon.