
You have undergone extensive training with one weapon that you choose, You gain the following benifits:
• Increase your Strenght or Dexterity score by 1, to a maximum of 20(dependant on which ability you use to wield the weapon you)
• Non-Magical disadvantage can't be imposed on your attack rolls
• You become proficient with this weapon, if you were already proficient with your chosen weapon you now have expertise with this weapon

Cool concept. but the "can't be disadvantaged except by magical means" is a bit unnecessary . A balanced substitute would be more like "once per long rest, you can use your deepened understanding with your weapon and attack at advantage for the turn" that way you can cancel disadvantage or give yourself advantage on command.