True Striker

You were born with booming natural power, inherited from your blessed bloodline, born from great warriors of the past such as Cù Chulainn, Hercules or the such. However, you were always told you needed to hone your power, just like those champions. As such, you went to a magic school along with any wizard worth their salt, and learnt the one spell you resonated with: True Strike. Yes, you dropped out shortly after, but it's not like all that practice is actually useful, right?

True Strikes

At 1st level, you have access to the one spell you have honed more than any other.
You learn the cantrip True Strike. It does not count against the number of cantrips you may know. Additionally, you gain proficiency with all simple melee weapons.

Magical Investment

At 1st level, you have learned to imbue your True Strikes with you own mana, increasing its power.
Whenever you cast the cantrip True Strike, you may spend a spell slot of any level to increase the bonus to the attack roll and damage roll by 1d6 per level of the spell alot spent. Additionally, you may use your spellcasting modifier instead of your melee attack modifier and your attacks count as magical for the sake of overcoming resistance and immunities when making an attack using the advantage granted by True Strike. The bonus to the attack roll increases to 1d8 at level 6, 1d10 at level 12 and 2d8 at level 18.

Empowered True Strikes

At 6th level, your training over your strikes have honed your skill, ensuring strikes reach their goal.
When you use the True Strike cantrip, you roll 3 attacks and use the highest roll instead of 2, and you deal a critical on a roll of 19 or higher. Additionaly, you ignore disadvantages due to darkness when using the True Strike cantrip.

True Inner Power

At 14th level, you have learned to infuse your weapon with your inner power, putting everything you have into a single strike.
As part of the action of casting True Strike, you may as well cast up to 3 spells that empower a melee attack, and apply them to the strike empowered by the True Strike cantrip. Additionally, you may use the highest level spell used this way as your spent spell for the Magical Investment trait instead of expending another spell slot. A counterspell spell cast on your strike targets one of the applied spells, to your choice, instead of stopping the strike.

Truth of the Blade

By level 18, you have mastered the blade in a way few may claim. Having honed your ability to strike, you are filled with the power of your legendary ancestors. 
When you obtain this subclass feature, choose between three legendary heroes as an ancestor: King Arthur, Cù Chulainn or Hercules. You obtain a boon from your bloodline depending on your ancestor.

King Arthur: When you use the cantrip True Strike with a spell slot of 1st level or higher and expend at least 1 spell to store in your attack, you may spend a fourth spell slot of level 3 or higher. If you do so, you erect a zone of protection in a 30ft radius around you. When a creature inside the protective area takes damage, you may nullify that damage by expending one of the spells stored in your blade of level equal to at least one tenth of the damage nullified, or spend one of your own spell slots of a level equal to at least 1 fifth of the damage nullified. You can no longer move when channeling your True Strike, but you gain resistence to bludgeoning, slashing and piercing damage. You may keep channeling your True Strike every turn using a bonus action, but you may only cast spells of the abjuration and enchantment school while channeling. Your Constitution increases by 2, to a maximum of 24.

Cù Chulainn: When you use the cantrip True Strikes with a spell slot of 1st level or higher, you may use a bonus action to temporarly forget a spell of 1st level or higher. If you do, you attacks using True Strike deal 1d8 extra damage, and on hit, the target must succeed on a Constitution Saving Throw of a DC equal to your spell save DC +2 per spell level or be poisoned for one minute per spell level until your next long rest. While poisoned this way, the target can no longer heal and has disadvantage on Wisdom (perception) checks to find your location. You may only use one spell of each level per long rest using this feature, and you may only use it a number of time equal to your proficiency bonus. Your dexterity score increases by 2, to a maximum of 24.

Hercules: Your unarmed strikes now deal 1d8 damage when using the True Strike cantrip with a spell slot of 1st level or higher. Additionally, when you hit using the True Strike cantrip in this way using your fists, you may spend a spell slot of 1st level or higher, and force the opponent to do a Constitution Saving Throw against your spell save DC or be stunned.While stunned this way, you may, still using the empowerment from the True Strike cantrip, use the same attack action to make a number of attacks equal to the level of the spell slot expended. If the target has less Hit Points than the spell slot spent times 10, it automatically fails the saving throw. Your strength score increases by 2, to a maximum of 24.