Ring of Life and Power

{1}When you roll a 20 on an attack roll, you may
use your reaction to draw on the power of this
magical ring. Immediately spend up to 4 hit
dice and roll 1d4 for each, gaining temporary
hit points equal to the total rolled.
{2}Alternatively, when another creature rolls a 20
on an attack roll against you, you may use your
reaction to spend up to 4 hit dice and roll 1d4
for each, reducing the damage taken by the
total rolled.
Requires Attunement.