The Black Knight

Your arms off!!

No it isn't!

Trial By Fire

Beginning when you choose this archetype at 3rd level, when your attacks score a critical hit you also deal an extra 2d8 necrotic damage as the sword combusts into blackflame, absorbing all light around it.

Additionally, when you roll a natural 1,  next attack shall be rolled with advantage, as the sword forces your arm to move in a certain way to be better.
Consequences of the unnatural movement of your arms the sword forces you to do are up to the discretion of the DM.

Broken Vessel

Starting at 7th level, your blade is now considered magical, and thus deals magical slashing damage, and acts as a +1 weapon, upon reaching 11th level it goes to +2, and then at 18th +3.

Additionally, due to the blades control of your muscles, and the harm you have succumb due to these unnatural movements, you become proficient in dexterity saves and gain expertise in acrobatics checks, you can also escape most manacles and restraints like grapples and such, you have advantage on all checks against being restrained and breaking out of those restraints.

Love For The Slaughter

At 10th level, You gain a number of "Curseblood" points equal to your profiency bonus.

You can expend one of these points to have your next attack be a "Brutality", where you tap into the insanity of your accursed sword and go for a sick and twisted slash that if hits, you may roll on the "Wounds and Scars" table, and inflict the result on your target.

You regain 1d4 Curseblood points on a long rest.

You have become so infatuated with your blade, you start going insane and must roll on the Short-Term Madness table after 10 minutes of not having it in your possession. Longer durations of not having it in your possession will lead to mental deterioration, long-term madness table shall be rolled after 1 day of not having it in your possession. Getting the sword back will remove short-term madness effects, but not long-term madness effects.

Bearer of the Blackflame

Starting at 15th, your blade constantly burns dark. You deal an extra 2d8 of necrotic damage on every hit, and you can use one action to absorb all light within a 30ft radius, acting much like the Darkness spell, centered on the blade. 

All checks to remember who you once were are made at disadvantage now. And you have an aversion to remembering, becoming hostile to whoever reminds you of your past before the black knight.

None Shall Pass!

You are fully attuned to your blade, and you have fully become the black knight. And as the black knight you are ruthlessly determined, you are immune to the unconcious condition, and when your hp reaches 0 the sword controls your body like puppet. You get a temporary +4 to all attack rolls and enemies have disadvantage to strike you.

The drawback is that the sword partly controls part of your brain. You also have extreme problems simply thinking for yourself, most desicions are decided by the sword and you are thus completely dependant on it.

Being seperated from the sword for longer than 1 week causes you to go completely braindead. You are unable to take any actions. Your speed is halved and enemies have advantage against you. The only way to cure this curse, is by either destroying the sword somehow. Or chopping the arm used to handle the sword, however, this shall reverse all progress and remove all levels of fighter from you.