You draw the life from every creature in a  cube centered around you the size of your watchblade range. Each creature in that area must make a Constitution saving throw. Constructs and undead aren’t affected, and plants and celestials make this saving throw with disadvantage. A creature takes 7d8 necrotic damage on a failed save, or half as much damage on a successful one. you heal 1d4 halved hit points  per creature affected in this way, rounding down.

Nonmagical plants in the area that aren’t creatures, such as trees and shrubs, wither and die instantly.

At higher levels: When you cast this spell using a spell slot of 6th level or higher, the damage increases by 1d8 for each slot level above 5th.