I suspect its probably referencing several things. Shiva is the obvious one. It could also be talking about a bond between dragon and man in general. We don't know what started the war and we cant be sure what came before. Perhaps the ancient Ishgardians once were allied with the Dragons and it was a betrayal most Ishgardians don't even remember that triggered the war. The lyrics make me strongly suspect the Ascians had a hand in the origins of the conflict.