Witchblade's Training.

Some covens of witches practice witchcraft not to sling spells, but to enhance their powers in battle. Witches who choose this way are called Witchblades, and are renowned for their supernatural adaptability in the battlefield. Like all witches, Witchblades gain their magical abilities by summoning spirits, but the way in which they do this is unique. They use their spirit-summoning abilities to imbue their blades with the spirits of warriors who came before them, learning their fighting techniques in the process. The stronger a Witchblade is, the more spirits they can summon to their weapon, and the more formidable and adaptable they are in battle. Switchblades excel in changing their tactics to suit their surroundings and exploiting their target’s weaknesses.
Witchblades are easily recognized by the weapon that gives them their name. Over time, reflecting its wielder’s growth in power, the weapon changes to reflect the particular brand of witchcraft of its wielder — growing in size, for example, or glowing. It is said that when a Witchblade grows sufficiently strong, their weapon itself becomes as changeable as its wielder, thanks to the spiritual energy constantly stored in it

Witchmark.

Witchblades’ witchmarks usually appear as scars. Instead of a singular scar growing as the Witchblade becomes more powerful, more and more scars appear along their body. It is said that the placement of these scars mirror the places where the spirits of the warriors they summon were dealt a killing blow. Other less common witchmarks for Witchblades include coats of arms in witch-knight orders, or swords that dance around Witchblades’ bodies when they use their witchcraft.

Level 3 Features:

Witchblade.

When you choose this archetype at 3rd level, your weapon becomes a beacon that pierces through the veil between the material and the Ethereal Plane, granting you the ability to contact spirits. You can transform any weapon you have proficiency with into your witchblade by meditating with it for 10 minutes. Your witchblade counts as a magical weapon for the purpose of overcoming resistance and immunity to nonmagical attacks and damage. Additionally, you always know the location of your witchblade as long as it’s on the same plane as you. You can’t have more than one witchblade. If you make a different weapon your witchblade, the former witchblade loses its magical properties and is no longer considered a witchblade.

Spirits of Masters.

At 3rd level, you gain the ability to summon a martial master to your witchblade, gaining their knowledge in battle. Select two martial archetypes from the list of martial archetypes available to fighters. After a long rest, you can meditate for 10 minutes to call for the help of the spirit of a master of one of these martial archetypes. The martial master spirit is bound to your witchblade, and you know all abilities unlocked at 3rd level of the selected martial archetype. For subclasses that allow you to cast spells, like the Eldritch Knight, and other subclasses that have specific resources to that subclass, you have the same resources they do at level 3. For example, a level 4 Witchblade Fighter that summons the spirit of an Eldritch Knight knows three spells, and has two level 1 spell slots. At the end of the following long rest, the martial master spirit leaves your witchblade, and you may choose to call upon the same or another spirit.
You can learn over time to call for master spirits of other martial archetypes. There are two ways to be able to summon a different martial master spirit. You can train for seven days with another fighter that has a martial archetype different from the ones you can call to your witchblade. After training, you will be able to call on the spirit of a master of that martial archetype. Alternatively, if you land the killing blow upon a fighter with a martial archetype, you gain the ability to call on the spirit of a master of that fighter’s martial archetype. You cannot learn martial archetypes from other witchblades.

Level 7 Feature:

Spiritual Style.

At 7th level, you are able to not only call on the knowledge of spirits for their martial archetypes, but for their fighting styles.
Select a fighting style other than the one you know. When meditating to use your Spirits of Masters ability at the end of a long rest, you also learn the additional fighting style you selected. At the end of the long rest, the fighting style leaves your witchblade and you may choose to call upon the same or another fighting style you’ve acquired.
Similarly to Spirits of Masters, you can learn to call for spirits to harness additional fighting styles. You learn to summon spirits of additional fighting styles in the same way you learn to summon spirit masters of additional martial archetypes. You cannot learn a fighting style and an archetype at the same time; if you train with or kill another fighter, you must choose whether to learn their fighting style or their archetype. Similarly, you cannot learn fighting styles from other Witchblades.

Level 10 Feature:

Battlefield Witchcraft.

At 10th level, you apply your knowledge of witchcraft and spirit manipulation to the battlefield, harnessing the spirits of those you have slain directly onto your witchblade.
When you deal the finishing blow on a creature that is hostile to you and of a CR equal or higher than 1, or when you land a critical hit, you can choose to gain one additional use of Second Wind. You cannot gain more than one additional use of Second Wind per turn. All additional uses of Second Wind gained through this ability disappear once combat is over and you are out of initiative. You can use this feature a number of times equal to your proficiency bonus. You regain all uses of this feature after finishing a long rest.

Level 15 Feature:

Exalted Witchblade.

At 15th level, the spiritual energy constantly stored in your witchblade changes it, giving it magical powers similar to your own.
As an action, you can transform your witchblade into the shape of any weapon you are proficient with. The witchblade retains only its colour, materials, and embellishments, and acts exactly as the weapon it has taken the shape of. If the witchblade had any bonus to attack or damage, it retains this bonus after transformation. The witchblade also retains any other magical properties it had before the transformation. For instance, if the witchblade was a +1 shortsword that dealt an additional 1d6 of radiant damage and is transformed into a warhammer, the witchblade is now a +1 warhammer that deals an additional 1d6 of radiant damage. If the witchblade is transformed into a weapon that requires ammunition, like a bow or a crossbow, you always have ammunition for this weapon. This does not ignore a weapon’s loading property or apply to thrown weapons like javelins, which have to be recovered once thrown.

Level 18 Feature:

Master of Spirits.

At 18th level, your connection with the masters you summon to your witchblade grows with you in power.
When selecting a martial archetype after a long rest through the use of your Spirits of Masters ability, you learn the level 18 feature or features of the martial archetype selected, in addition to the level 3 features. For subclasses that allow you to cast spells, like the Eldritch Knight, and other subclasses that have specific resources to that subclass (that you unlock at either level 3 or level 18), you have the same resources they do at level 18. For example, a level 19 Witchblade Fighter that summons the spirit of an Eldritch Knight knows 12 spells, and has four level 1 spell slots, three level 2 spell slots, three level 3 spell slots, and one level 4 spell slot.