Boon of Deific Vitality

Through some sort of mechanic whether that be divine intervention, overexposure to some kind of magic or even an artifact, you have been blessed with godlike vitality.

Supernatural Durability: All dice rolled to determine total hit points are maximized, this is retroactive. In addition, anytime you're granted temporary hit points that require a die roll, you instead get the maximum amount.

Regeneration: You gain the ability to regenerate wounds. The rate of regeneration is equal to your character level divided by 4, rounded down (minimum of 1) per round. Should you be reduced to 0 hit points or less, you fall unconscious. You automatically pass any death saving throws until you become stable. An effect that stops regeneration such as Chill Touch, can stop this regeneration, forcing you to make a death saving throw for that round. If you fail 3 death saving throws before rising above 0 hit points, you die.

• Severed limbs: Limbs and digits separated from your body do not regenerate. However if you still have the dismembered limb and can place and secure it where it's supposed to be while you heal, it will resume full function at the end of the round.
• Severed head: If your head is separated from your body, you stop regenerating and immediately start failing death saving throws. Should one of your teammates spend a round to hold your head to your neck in proper orientation (Medicine check, DC 10) before you die, it will reattach at the end of the round.
• Short and Long rests: As long as there's no effect stopping regeneration, all your Hit points are healed after an hour.  This is strictly a time-based effect and will continue to function even in areas where it's impossible to rest.
• Exhaustion: You have advantage on saving throws made to resist exhaustion. On a failed save you still suffer the normal effects of any exhaustion levels you have. As exhaustion levels represent both mental and physical fatigue, all the normal rules apply for recovering from the effects of exhaustion.

Immutable Form: You have advantage on Constitution saving throws made to resist poisons, diseases, and constitution-based effects that would change your body, such as mutations.  Also, any such effects are reversed at the rate of 1 per hour if you aren't engaging in strenuous activity (walking, reading, etc.)