Answer:
The Acrostic will use the word "Soulful" - to express deep feeling with strong emotion.
Explanation:
Two of the couplets rhyme, the others are blank verse.
S - Saddest song to ever be breathed,
O - Onto the hardened heart,
U - Understood by no one, the anger seethed,
L - Last to leave, last to hear, last to love.
F - Finally though, the soulful song, touched the heart, so savaged.
U - Unloaded at last, hurts all past,
L - Love now free, love unbounded, love flying above.