+2 Con, +1 Dex

the Chair can be equipped by any other creature/character that can weld 2-handed weapons (with or without the Chairs will).

While equipped the Chair is a 2-handed melee weapon, which inflicts damage equal to D12 + the total STR modifiers of both the user and the Chair combined.

However, whenever the weapon inflicts damage, the Chair must pass a CON saving throw of a DC equal to 10 + the users strength modifier - if failed the chair is also damaged equally from the attack.

While equiped, the Chair can only try to either unequip itself as an action (Str roll off between itself and user), or attack the user as an action. If it becomes unequiped during a round, it's movement is halved for its turn.

on each full moon the chair witnesses, it must pass a DC15 CON save or else transform into a rabid werechair for the remainder of the night (a hairy, toothy chair). If passing the CON save, the chair still becomes a werechair, but has free will and can continue to act as normal.

A rabid (i.e. CON-save-failing) werechair will bite and attack anything in its area. 

As a werechair, the character has +2 to Str and Dex, and -2 to Wis, Int, Cha. 

Any ordinary chair bitten will become sentient as a werechair also, with powers and strength roughly equal to half of the chair that bit it, and is "born" rabid.