You heal a creature you touch at your own expense. Spend one of your own hit dice. Roll it and add your Constitution. You take necrotic damage equal to the total (this cannot be reduced in any way) and the creature you touch heals the total. Undead and constructs cannot cast this spell, nor does it have any effect on them. 

At Higher Levels. When you cast this spell with a spell slot of 2nd level or higher, you may spend one extra hit dice for each slot level above 1st. Add your Constitution to each dice roll.