Crimson Blade

You cut open your own skin, and the blood that flows out of the wound forms a one-handed light melee weapon of your choice in your hand. You can't be disarmed of this weapon, and you can use a bonus action to summon it in your hand. You can also use your spellcasting modifier for attack-and damage rolls with this weapon. The weapon deals 2d10 Necrotic damage on a hit, and half as much damage on a miss. Whenever you kill a creature with this weapon, it grows stronger; it deals an additional 1d4 damage for every creature that is killed with it, and gains a +1d8 bonus for every 3. This weapon also has the Nick property automattically unlocked. When this spell is cast, you also automatically make one attack with this weapon.

At higher levels: The weapon deals +1d10 damage for every spellslot level above 4. Aditionally, if you expend a level 7 or higher spellslot, you can summon two at the same time (one in each hand).