Essence Drain

You can use magic to transfer life essence from another creature into yourself. Point at a creature within range. The target must make a Constitution saving throw. On a fail, the target takes 2d4+1 force damage and loses half as many hit dice, or half as much damage and does not lose any hit dice on a success.

You then gain a number of temporary hit points equal to the damage it took, and a number of hit dice equal to the number it lost.

At higher levels. When you cast this spell with a spell slot of 3rd level or higher, the force damage increases by 1d4 for each slot levels above 2nd.