Guardian of the Cursed Weapon

Warlock Subclass

A Guardian is holder of a powerful weapon, and has the responsibility to keep it under control and protect those around them. This can prove difficult, and in some cases impossible. Through determination they learn to either conquer their demon or work alongside it as an ally.

Feats

A Guardian's most important possession is their weapon, a source of their power, a symbol of their duty, and the most dangerous thing they carry. A Guardian without their weapon is powerless, and even worse the people around them are at risk. Any non Guardian that makes contact with a Guardian weapon that has been bonded will become possessed by the demon completely. The demon only becomes free if it can kill the Guardian with its own hands, however if the Guardian dies while its contained, it will perish aswell, forcing the demon to have a vested interest in its Guardian's survival

Tainted Power

The weapon a Guardian carries contains a demon, and as such this weapon is a dangerous conduit of power that will grow with their own power. The weapon is counted as a magical weapon for resistance purposes and the weapon its inhabiting has your lvl x 2 added to its damage total. The Demon can inhabit different weapons and must choose to move weapons, but each has its preferred weapon. Guardian warlocks have no spell slots and instead have nearly unlimited access to their spells, but each cast has a chance to let the demon take control. Whenever you cast any spell the you must roll a D20, on a 3 or lower the demon posses you for up to 25 - lvl hours.

Careful Control

After learning to carefully control your casting, at level 6, you can cast a spell without making a possession roll as many times as your charisma bonus

Overcharge

you can channel the demon's power with ferocity, at level 6 you can double the rolled damage of an attack at the cost of a possession roll as many times as your charisma bonus

Symbiosis 

Depending your relationship with your demon you can take one of the following effects at level 10:
If your bond with the demon is positive, the DC for possession is raised to 4 and possession time is halved, but once per long rest you can assume the form of the demon for 4 turns and have unfettered access to your spells, aswell as the Demons own abilities  
If your bond with the demon is negative, the DC for possession is lowered to 2, and twice per long rest you can cast any of the warlock spells at a heightened level and as a bonus action (Free action if it already a bonus). Additionally you can Overcharge one additional time per long rest with no possession roll 

Keeper

Depending on your previous Feat you gain one of the following effects at level 16:
Once per long rest you can manifest your demon to fight along side you for as many turns as twice your charisma bonus and while manifested, you gain half of its health as temporary HP
Twice per long rest you can dismiss any demon that is below you in level or bend it to you will for as many hours as your charisma bonus. the DC for possession is lowered to 1