Soul-Binding

When dealing a killing blow (an attack that reduces a creature to 0 hp or less) you may use you reaction to attempt to to claim and detain that creatures soul. You must make a contested Con saving throw. The DC is the creatures Con saving throw plus its proficiency bonus. On a successful save, you capture and bind the creatures soul to an object on your person. On a failure, you take necrotic damage equal to a number of d8's equal to the creatures Con modifier and your max hp is reduced by the same. If this damage would reduce you below 0 hp, you soul is instead bound to an item of the creatures you attempted this ritual on. 

At the time of a contested roll, you may expend a warlock spell slot to add a bonus equal to the level of slot expended to your roll. You may do this after you have rolled but only before the results have been announced. 

Additionally, you may declare a duel with a creature who has not taken damage or attacked another target at any time. If you attempt to Soul-Bind a creature you are dueling, that creature has disadvantage on their Con saving throw. However, if at any point during the "duel" you or a dueling creature attack or are attacked by a creature outside of this duel, this condition is lost.