Path of the Rune Keeper

A guardian of ancient magics who wears upon their flesh the lost and most ancient of magic forms. Long exposure to violent untamed magic and the barbaric ritual of carving Runes into their very flesh has left the Rune Keepers with no fear of pain. Violent surges of untamed magic drive them to explosive action, and dangerous magic surges threaten to destroy their body and mind.

Runic Vitality

Starting when you choose this path at 3rd level, you can  channel raw magic energy  through the Runes carved into your flesh at the expense of your bodies stability. When you use the Rage feature,  you may designate a value of your current hit points to convert into temporary hit points. Your maximum hit points are  lowered by the value of the temporary hit points acquired. Your hit point maximum can only be restored by a short or long rest. 

While you have these temporary hit points,  you deal 1d6 extra damage on all melee attacks. The extra damage is force damage. This damage increases to 2d6 at level 11.

Volatility

Starting at 6th level,  the raw magical energy that fuels your rage infuses itself into your attacks. While you have temporary hit points granted by the Runic Vitality feature, you may, as a bonus action, forgo the benefits of your unarmored defense and instead add half the value of your proficiency bonus--rounded up--to any attack or damage rolls until the start of your next turn.You may use this feature a number of times equal to your constitution modifier. Once expended this ability resets on a short or long rest.

Shield of Chaos

At 10th level, while you have temporary hitpoints from the Runic Vitality feature, you gain resistance to damage from spells. 

Runic Revenge

Beginning at 14th level, the chaotic energy of the raw magical power that courses through you is at its wildest. Whenever you have temporary hit points granted by the Runic Vitality feature, and are struck by a melee attack, the attacker takes damage equal to your constitution modifier.