Staff of the Unyielding Oak

These staffs, of which three are known to exist, are treants that have willingly bound themselves into quarterstaff form to aid Obad-Hai's followers. This ability functions like the changestaff spell, except that the treant is fully real and can control trees and speak to other treants. If the staff of the unyielding oak is reduced to 0 or less hit points, it reverts to staff form and can't be used again for twenty-eight days.
The staff can take treant form as often as you like, but it must spend at least half of each day (dawn to dawn) in staff form.