Treehead Woodfist is a Super Unique Brute found in the Dark Wood guarding the Tree of Inifuss in Act I of Diablo II. Killing him is optional as the Search for Cain quest can be finished without doing so. However, avoiding him and his gang may be difficult as they move 'extra fast'. Separating Treehead from his minions won't be easy either.
It's best to avoid being cornered by Treehead at all cost.