Lost In a Dream

A world nobody knew that existed. Until our MC started to dream about it. He also met a person in that dream. Slowly he learns about that world. But later on he saw how that person dies and that world falls apart. He wishes to help him but he thinks that it's just a dream, but little did he knew that he was going to fall in that dream and as that person. But he also doesn't want to die tho. Now, how is our MC going to survive?