Energy Theft

You imbue your weapon with chants of inversion and negation, then strike out and take an enemies natural strength and make it your own.  As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature and hit, otherwise the spell fails and the spell slot is wasted.  On a hit, the target suffers the attack’s normal effects plus an additional 3d8 necrotic damage, and if the target has resistance or immunity to acid, cold, fire, lightning, necrotic, poison, psychic, radiant, or thunder damage it loses its resistance or immunity to one of the above damage types (your choice) and instead gains vulnerability to that damage type.  Additionally, you gain resistance to that damage type for as long as the target suffers the above vulnerability.  

The target can make a Constitution saving throw at the start of each of its turns, ending this effect on a success.  If the target had immunity to the chosen damage type rather than resistance, this save is made with advantage.  This effect also ends if you lose concentration, if the target dies, or if you become incapacitated.  

At Higher Levels: When you cast this spell using a spell slot of 3rd level or higher, the extra necrotic damage increases by 1d8 for each slot level above 2nd.  Additionally when you cast this spell using a spell slot of 4th level or higher, you may choose one ally to also gain the same resistance that you do from this spell.  You may choose two allies to gain this benefit when you use a spell slot of 6th level or higher, and three allies when you use a spell slot of 8th level or higher.  An ally must be within 30ft of you when you cast his spell to gain this benefit.