Seraph archetype

The seraph makes fatall attacks on the battlefield at the cost of ones on health. They work best when in danger and instingtivly knows how to deal with ceartian situations specifically when under stress. The pain you feel from taking a hit is used to draw out strength never imaginable. The seraph is even capable of drawing flames out of thier body, a fantastical feat performed by ones sheer willpower and belief. 

The source of ones flames are unkown by all, some say it is a boon bestowed by a mysterius god, others think it is a materilized version of the inner battle spirit formed by the fighters feral instincts, or maybe it is a devilish curse that only eats away on people from the inside. The source is vauge and really it is up to the players/DMs to decide. 

Beast mode

After you choose this archetype at 3rd level, you gain a beastial sense that makes your attacks more fatall as you lose health. Whenever you take damage during your turn, you deal an additional 1d8 fire damage on top of your melee attack damage roll, this only stacks up to one times, and is active for that turn.

The damage bonus turns into a 1d10 when you reach 15th level.

Life drain

At 7th level, if you are under maximum hit points, the strain on your body will worsen your condition as you perform attacks. Dealing damage by making a melee attack like this will cause you to take dmage. You take 1d4 fire damage. (You dont gain the bonus bestowed by "beast mode" the bonus is added on all subsequent attacks). If you wish to not take damage, you can make a CON saving throw once at the start of your turn. If you fail the save you still take damage, but upon succeding you dont take any damage from this skill, for that turn.

Additionaly you are now resistant to fire damage. 

Got the guts

At 10th level the inner flams of your body build up over the course of a fight. When your hit points go under half of your maximum number of hit points you may once per turn deal an additional 1d6 fire damage on your melee weapon damage rolls. Once again when your hit points drop bellow 20 you may instead deal 2d6 fire damage.

Inferno dance

Starting at 15th level once every long rest you may burst the fire from within, dealing masive damage when the flames have more wounds on your body to exit through. You attack all creatures in a 15ft cone in front of you, those creatures must make a CON save, if they succed they take half damage. otherwise the burst will deal 4d6 fire damage, however if you are below 75% of your health you add 2d8 radiant damage. If you perform this action when below half of your health you deal 2d10 force damage as well. And once you do this action when below 25% of your health, enemies take an additional 2d12 fire damage

Universal Annihilation

At 18th level your battle hardened soul has earned you a sharp mind. As long as you are below 20 hit points in battle you have advantage on all attack rolls, and all damage you deal count as a critical hit.