cannot expect the metal song's mood to be very joyful or merry). About the person who is trying to get back his love. The first few stanza's describe her being very beautiful with she being not-so-happy with his arrival (No joy would flicker in her eyes, brooding sadness came to a rise). Even words would fail to repair the situation then as he realizes that she is no longer single and is already committed (probably married) to someone else. (Words would falter...She had sworn her vows...). But he still wanted to have her back, and tries that to have some contentment (probably that of atleast trying before giving up. Not sure though). And so he asks her finally, if she can come back with him, as he has come back only for her. And then in the midst of describing her situation like being very much confused and baffled comes her epic reply: "My promise is made but my heart is thine", indicating that she is already committed (socially it should be so I guess marriage it should be), but her still belongs to him.
Like I said, many people (or couples) do not get to always stay together, despite having mutual feelings for each other. This may be due to social, parental pressures and so on. There might be numerous reasons and likely hasn't been portrayed in the song as well. But the emotions involved have been captured brilliantly.
Man, what a song! This is just so amazing, not only the lyrics are awesome, but has some really divine music. Each musical piece suits the lyrics appropriately. Calm and mild while describing her and some very turbulent pieces, when we see him re-asking her.
Beautiful song!