Eldritch Blade

You channel raw eldritch power into your weapon and make one melee weapon attack against a creature within range.
On a hit, the target suffers the weapon’s normal effects, and the attack deals an extra 1d6 force damage as crackling eldritch energy surges through the blade.

Level 5 unlocks a new ability: Overkill Surge

Overkill Surge-
If this attack reduces the target to 0 hit points, the excess eldritch energy violently discharges. Choose one of the following effects:

Burst Option-
Each creature of your choice within 5 feet of the target must make a Dexterity saving throw on a fail they take force damage equal to double your spellcasting modifier and half on a success.

Pierce Option-
The remaining energy lashes forward in a line. One creature directly behind the target (within 5 feet, in a straight line) takes force damage equal to your spellcasting modifier.

Scaling:
At higher levels, the extra force damage increases:
5th level: 2d6
11th level: 3d6
17th level: 4d6

This cantrip can also be gained by any feat, or background that gives access to warlock cantrips.