Ashes of the Ages and Eternal Fire (jade humanoid bicuspid)

You can use the tooth to cast counterspell at 9th level. Once you use this property, it can't be used again until the next dawn. Whenever you finish a long rest, if you haven't used the tooth to counter a spell since your last long rest, your hit point maximum is reduced by 2d10. If this reduces your hit point maximum to 0, you die.