Dorothea's Draining Blade

You take the dagger used in the spell's casting and instill it with vitality draining energy, and the blade of the dagger is shrouded with blood-red arcane mist. While you maintain concentration on the spell, you can use an action to make a melee spell attack against a creature that you can see within 15 feet of you, as you slash out with the dagger and a bloody tendril shoots out and attempts to pierce through the target. On a hit, the creature takes 4d12 necrotic damage and must succeed on a Constitution saving throw or the creature's Strength score is reduced and your Strength score increases (to a maximum of 30) by 1d4 while you maintain concentration on the spell, as the creature's power is transferred to your own. If you reduce a creature's Strength score to 0 by using this spell, the creature becomes incapacitated until you break concentration. If you kill a creature using this spell, all bonuses to your Strength score that you have drained from the creature are maintained until your next long rest. This spell has no effect on constructs or undead.

At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, the damage increases by 2d12 and the Strength drainage increases by 1d4 for each slot level above 5th.