The Nettlebane is an artifact dagger which can be acquired by completing the quest The Blessings of Nature, given to you by Danica Pure-Spring, a healer in Whiterun. The quest will first send you to Orphan Rock, a rock inhabited by a Hagraven and a few witch followers. Upon killing the Hagraven, you'll be able to obtain her Nettlebane.
When the Dragonborn returns the dagger to Danica Pure-Spring, she suggests you take it to the Eldergleam Tree in the Eldergleam Sanctuary, as she's busy tending to the weak in her chapel. Upon agreeing, the Dragonborn must take the dagger to the Sanctuary. Once there you're informed that the dagger is the only thing strong enough to break the roots of the Eldergleam tree, granting passage to the tree itself.
The Nettlebane will inflict extra damage on supernatural foes and those without protection against enchanted weapons.