So people complain about melee squishing us? here's a fix.
Basically it's for casters. The higher of it you have, the higher chance that melee attacks will hit your magical~ armor and be lost in the void of spells and arcane and such. The melee damage is then either lost or could be redirected back to the attacker (maybe 25% of it)
This could also be a general buff to all spell casters. You start off with 1 void absorption, and the more damage you take, the higher it stacks (up to 10 or so) At 10, you can cast a spell to unleash it all at once.
Any takes done to you will be lost or redirected. Stacks are gained with damage taken and persist through death.