Eternal is my favourite instrumental piece. I love the thunder. Eternal begins anxiously. I think it describes hardships in somebody's life. He or she is anxious and scared and doesn't know what to do. The situation becomes harder all the time until it explodes. The thunder means a dramatic change of life, maybe a fight. Music fades out, you can only hear thunder and rain beginning. After that the music is back again but it's more peaceful now. The fight and hardships in his or her life are over and a better period begins. There's still the same basic tune that goes on the most of the song. That means that everything in life doesn't change.
I think the name Eternal means the eternal circle of life. Good and bad periods follow each other in turns but life goes on.