Soulstealer

A blacksmith can forge steel into a weapon, an artificer can imbue that weapon with properties, but only those who train in the art of soulforging can use the power of a creature's soul to make their weapon more deadly. Many apprentice to a soulforge master to hone their skills and even sell the weapons they create. Others may serendipitously discover it on the path of the adventure. Regardless of how you have encountered the art of imbuing a weapon with a soul, it will serve you in your battles.

Soulforge

You can sense the souls of creatures leaving their body, and have the power to pull them into an object.

When you see an enemy reduced to 0 Hp or fewer, you may use your reaction to trap their soul in a melee weapon you are wielding, improving it. The chosen weapon gives off a faint purple aura, and becomes magic for the purpose of overcoming resistances. It receives a +1 to damage and its damage becomes necrotic. The weapon loses this effect at the end of the current combat, or if the creature is resurrected. This feature stacks and you may use it a number of times equal to half your fighter level per short rest.

Prevent Death

Your skills in soul magic allow you to protect a friend, moving their soul back into their body. You may cast spare the dying as a cantrip, but do not have to touch the creature, instead it can be a creature you see within 30 ft. You may use this feature twice per long rest, and three times per long rest once you reach level 15.

Improved Soulforge

Your work with souls has allowed you to feel them not only when they are separated from their body, but when they cry out in pain too. You may use your soulforge after successfully dealing damage to a character, pulling their lifeforce out of them and into your blade. This still counts as your reaction for the round. The creature visibly ages appropriately for the damage they received. Your soulforge now gives you a +1 to attack as well, and any damage die that is a 1 is treated as a 2. 

Improved Reaction

Once you reach level 10, you get 2 reactions per round.

Command Spirit

While you have your weapon imbued with Soulforge, you may spend an action to release the soul, ending its effect. Its spirit appears and you can command it once before it dissipates, to move and attack. The spirit resembles the creature who died, but moves and makes a withering touch attack, as if it were a ghost. 

Versatile Soulforge

Your combat prowess with souls grants you an increased ability to command the essence of the souls you have forged. As a bonus action, the purple necrotic aura can take the shape of a different weapon, changing the weapons damage die. The new weapon must share one of the properties of the weapon you are wielding. The damage type is still necrotic. You may use this feature a number of times equal to your wisdom modifier before a long rest.