death knight

death knights made a deal with death itself in exchange for dark powers, they can summon a spectral weapon of death energy and even raise undead servants to fight for them. however, death will claim them soon, no matter the fight they put up.

spectral armament

you can call a weapon made of death magic from the abyss. at third level as a bonus action on your turn, you may summon a death weapon, this takes the form of a weapon of your choice and counts as magical for the purpose of effects, in addition is deal an additional 1d6 necrotic damage.

dead minion

your power over death has grown to allow you to raise the dead yourself. at seventh level once per day you may cast animate dead as a fourth level spell, this recharges on a long rest.

improved spectral armament

your spectral gears power has grown. at tenth level you add +2 on to hit rolls with your spectra armament and it deals an additional 2d6 necrotic damage instead of 1d6.

king of the dead

the undead recognize you as a servant of death. at fifteenth level undead will not attack you unless compelled by a magical force, lich's and other sentient undead ignore this. 

angel of death

you might as well be death itself. at eighteenth level whenever you would score a critical hit you may suffer one level of exhaustion to deal necrotic damage equal to the targets health, this damage ignores resistances and immunities and cannot be blocked in any way. (Note to DMs: give your big bad a second form ;).