** This is intended as a 10th Level Spell for a Dante's Inferno style homebrew campaign. **

THE GIFT

You fly into an infernal rage, taking on aspects of the Archdevil Zariel. For the spell duration, you gain the following abilities:

• Your Speed increases by 50ft. You gain a Fly Speed equal to your walking speed as a pair of eternally embering black, feathered wings sprout from your back
• You Do Not Provoke Attacks of Opportunity
• You have Advantage on Saving Throws against Magical Spells and Effects
• You gain Immunity to Fire Damage
• You gain Resistance to All Damage
• You gain One Extra Attack

Additionally, your weapon takes on the aspects of Zariel's Blade. Your Weapon's Enhancement Bonus is equal to +5. You deal an additional 2d10 Hellfire Damage on a successful hit (bypasses fire resistance, half damage against fire immunity). On a critical hit, your additional damage increases to 8d10 instead of doubling.  Additionally on a critical hit, Hellfire spreads around you in a 30ft radius, dealing 2d10 Hellfire Damage to all enemies within range and setting them on fire (1d10 Hellfire Damage per round until extinguished). A successful Dexterity Saving Throw (DC = Spellcasting DC, +2 if on Avernus) halves the damage and prevents them from catching fire. 

If you cast this spell while on Avernus, the duration extends until dismissed and does not require concentration. If you leave Avernus with the spell active, the duration returns to 1 hour, and does not reset or extend if you return to Avernus in that time.

THE PRICE

When the triggering damage takes place, your Hit Point Maximum is Reduced by that amount as part of the price for casting the spell. This cannot be restored in any way until after a Long Rest, except by the Wish spell or similarly powerful magical effects. 

When this spell ends, you must make a Constitution Saving Throw. You cannot use the advantage gained from this effect for this saving throw. The DC for this saving throw is equal to your Spellcasting DC. On a successful saving throw, you suffer Two Levels of Exhaustion as your insides strain against your skin from the stress. On a failed saving throw, you suffer Five Levels of Exhaustion and Fall Unconscious for 1d4 days as your blood begins to boil, your bones snap, and your muscle tissues tear themselves apart. You cannot be awoken or have your exhaustion removed by any means short of a Wish spell or similarly powerful magical effects when you fall unconscious in this way. Your Exhaustion starts dissipating as normal after you awaken from unconsciousness. You gain Advantage on this saving throw if the spell ends while you are in Avernus.