Fighters who adhere to the Ardent Knight archetype are those who, perhaps through esoteric ritual or through natural mutation, has developed the ability to channel heat through their bodies and use it to augment their combat abilities. Those who can master the flow of heat make for terrifying masters of battle, allowing them to burn their foes with their blade and enhance their hardiness in battle. To face an Ardent Knight is to know the might of one's inner flame made manifest, and to meet a warrior that few could ever hope to defeat.

Strength of the Inner Flame

When you choose this archetype at 3rd level, you gain proficiency in the Athletics skill. In addition, when you make a Strength ability check or saving throw, you can use the heat within your body to magnify your strength, granting you advantage on the roll. You can do this a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a long rest.

Calefaction

At 3rd level, you learn the basics of augmenting your combat abilities with the heat in your body.  Beginning when you gain this feature, you imbue any weapon you are holding with heat which causes it to glow red-hot, causing it to deal an extra 1d4 fire damage. This extra damage increases to 1d6 at 5th level, 1d8 at 11th level, and 1d10 at 17th level.

In addition, you can absorb the heat of your fallen enemies to bolster your own strength. When a Small or larger-sized creature you can see within 15 feet of you dies, you can choose to absorb the heat from the creature's body (no action required), generating a unit of Heat within your body. The creature must have held heat in its body prior to dying, and you can only hold a single unit of Heat within you at any given time.

As a bonus action, you can expend one unit of Heat to to grant yourself temporary hit points equal to 5 + your fighter level.

Invigoration of The Fervid

Starting at 7th level, you develop a way to utilize heat to further empower your abilities. As a bonus action, you can expend one unit of Heat. When you do so, you gain resistance to fire damage, and the extra fire damage dealt by your Calefaction feature ignores resistance to fire damage. This effect lasts for one minute or until you end it (no action required).

Thermal Resilience

Starting at 10th level, your body's inner temperature now burns away all impurities. You gain resistance to poison damage, and are immune to all diseases and poisons.

Heat's Soothing Incandescence

Starting at 15th level, your body has become adjusted to withholding extreme heat, so much that absorbing it now heals you. Your body can now hold up to two units of Heat within it at any given time. In addition, whenever you generate a unit of Heat, you regain hit points equal to half your fighter level (rounded down).  You cannot exceed your hit point maximum by regaining hit points in this way.

Pyrexian Endurance

Starting at 18th level, you can use the heat into your body to save yourself from death. Once on each of your turns, if you drop to 0 hit points and don't die outright, you can immediately expend a unit of Heat to drop to 1 hit point instead. When you do so, each creature within 5 feet of you takes 8d6 fire damage.