Band of Iron Thorns

This black iron armband bristles with long, needle-sharp iron thorns. When you attune to the armband, the thorns bite into your flesh. The armband doesn't function unless the thorns pierce your skin and are able to reach your blood. While wearing the band, after you roll a saving throw but before the GM reveals if the roll is a success or failure, you can use your reaction to expend one Hit Die. Roll the die, and add the number rolled to your saving throw.