Scion's Training.

Whether because they belong to a long lineage entrusted as stewards of a centuries-old artifact, or because a mysterious relic is thrust upon them by happenstance, a Scion is a Fighter in possession of an extremely powerful weapon.
Scions use a mythic arm: a weapon brimming with magic and, just like the Fighter who wields it, potential. It’d be a mistake to assume that a Scion is only strong because of the weapon they build. To actually access the power of the mythic arm, a Scion must hone not only their technique, but their connection with their weapon.
By strengthening the bond between the two, the Scion becomes the only one able to harness their weapon’s true power and unlock its full potential.

Level 3 Features:

Mythic Arm.

You wield a mythic arm, an extremely powerful weapon brimming with boundless potential for you to unlock. How you acquired this mythic arm is up to you, but it must be a weapon you are proficient with.
When you reach 3rd level, choose one of the following damage types: Acid, Cold, Fire, Lightning, Necrotic, Poison, Radiant, or Thunder. All mythic arm attacks made with the additional action granted by your Action Surge deal 1d8 of your chosen damage along with your weapon damage.

Wielding Legend.

The powerful magic that makes your weapon a mythic arm binds it to you and protects it from tampering. Your mythic arm counts as magical for the purpose of overcoming Resistance and Immunity to nonmagical attacks and damage and cannot rust, be dulled, or break. You cannot be disarmed while wielding your mythic arm as long as you’re not incapacitated.
You can also sense the direction of the mythic arm’s location and on which plane of existence it’s located. If the arm is moving, you know the direction of its movement. Additionally, your mythic arm can absorb the enchantment and the shape of another weapon. When you acquire an enchanted weapon, you can spend one hour to transfer the enchantment from the enchanted weapon into your mythic arm. For example, if you wield a Longsword as your mythic arm and acquire a +1 Handaxe that has Advantage on all attack rolls against Beasts, you can take one hour to transfer that enchantment to your mythic arm, after which it will be a +1 Longsword that has Advantage on all attack rolls against Beasts.
You are also able to change your mythic arm into the shape of any weapon it has previously absorbed an enchantment from by spending one hour concentrating on it. For example, after absorbing the enchantment from the +1 Handaxe of the previous example, you can now turn your mythic arm into a Handaxe by spending one hour concentrating on the mythic arm. If you do not meet the prerequisites to attune to the weapon, you cannot transfer its enchantment into your mythic arm. Once you have transferred an enchantment into the mythic arm, the original weapon loses its enchantment.

Level 7 Feature:

Wondrous Wielder.

You gain further control of your mythic arm, and you can manipulate the way you wield it in battle to transcend the limits of normal weapons. At the end of a Long Rest, you can select one of the following weapon properties to apply to your mythic arm: Finesse, Light, Reach, or Thrown (Range 20/60). The mythic arm retains the weapon property indefinitely, until you use this feature to select a different property.

Level 10 Feature:

Unlocked Potential.

The bond between you and your mythic arm allows you to draw even more power from your weapon:
The additional damage you deal when making an attack with your mythic arm with the additional action granted by your Action Surge increases to 2d8. At 18th level, it becomes 3d8.
You choose an additional damage type for your Mythic Arm feature. When you use an Action Surge, you choose which of the available damage types your additional attacks will deal. At 18th level, you can choose one additional damage type.
You can absorb an additional enchantment into your mythic arm. If more than one enchantment includes a bonus to attack and damage rolls, the higher bonus is applied to your mythic arm. At 18th level, you gain the option to absorb yet another additional enchantment, to a total of 3.

Level 15 Feature:

Unleash Potential.

You gain further control of your mythic arm, allowing you to unlock some of its hidden potential without exerting yourself in the process. On a successful attack, you can deal the additional damage you would deal with your Action Surge attacks. You can use this feature only once per turn.

Level 18 Feature:

Mythic Scion.

Your mythic arm has become an extension of yourself, unlocking its full potential. Whenever you make an attack roll with the additional action granted by your Action Surge, you treat all rolls of 9 or lower as a 10, and those attack rolls cannot be made at Disadvantage.
Additionally, when you roll for Initiative and have no uses of Action Surge remaining, you regain 1 use.