Wondrous Item, rare (requires attunement by a (requires attunement))
This black iron armband bristles with long needle thin thorns of iron.
When you attune to the item the thorns bite into your flesh. The armband does not function unless it is actively penetrating your skin and has access to your blood.
While wearing and attuned to the band, after you roll a saving throw but before the DM gives you the results, you may use your reaction to expend one Hit Dice. Roll the die, and then add the number to the saving throw.
Notes: (requires attunement)