Chair of Doom

You point at a chair and speak the incantation c̵̲̹̓̚ḩ̷͉̅̈́̂͐̊ȧ̶̠̆̏͂ỉ̴̖̻͇̑́̓r̶̨̭͓̕. The target chair then comes to life and begins to fight for you.  You can choose the stat block of any beast or monstrosity the chair's size up to a CR of 12 with these differences: the chair has an AC equivalent to its material + it's dexterity; it does not have a flying, swimming or burrowing speed, but t can have a climbing speed;  it cannot speak and it has 60ft of blindsight. This lasts for one hour or until dispelled. If this chair is a throne made of a valuable material, it's maximum CR is 15.