Well, we know that obsidian, or “dragonglass,” can kill White Walkers, so it’s a fair guess that there’s some link between it and Valyrian steel. However, thus far that’s all we know. So, is Longclaw the sword that will defeat the White Walkers, and by extension, does that make it Lightbringer?
It’s a good question. We don’t really know much about Longclaw’s origin beyond the fact that it was the Mormont family blade. However, it’s unlikely that it was forged by being plunged into someone’s heart, as Lightbringer was. So it could easily be the blade described in the stories about the Last Hero, but it probably isn’t Lightbringer. But here’s the thing: for the prophecy to make sense, Lightbringer doesn’t have to be a sword.