Book about an aging emperor seeking immortality/youth from an enchanted forest. He sends his military into said forest to seek the secret/potion/magic from the hidden forest dwellers.

[Ibukun]

Hello, I need help recalling the title of a book I read about 15 years ago (I think around 2008).

Plot: An aging/dying emperor seeks immortality from an enchanted forest. He sends his military into said forest to seek the secret/potion/magic from the hidden forest dwellers. The forest dwellers presumed to have the secret to eternal youth are somewhat 'fairy-like' or 'elven-like' - in any case, not 'strictly human' although they do have external human appearance (from what I remember). The soldiers are to recover this key to eternal life at any cost. Our main protagonist is an officer of the emperor's army. There are secondary key characters like the emperor's confidante, the emperor himself and a member of the forest dwellers.

Genre: Fantasy, military -- could be considered a bit of 'science fiction' because of the idea of anti-aging formulas. But I would predominantly consider it fantasy.

Thank you!