Life Siphon

9th-level Necromancy

Classes Cleric, Sorcerer, Warlock, Wizard

Casting Time: 1 action

Range: Self

Components: V, S

Duration: Instantaneous

Tendrils of necromantic energy spiral off you and leach the life force from nearby creatures. Choose up to six living creatures within a 120-foot-radius sphere centered on you. Each target must succeed on a Constitution saving throw or take 8d6 necrotic damage. For each creature that takes damage, you regain hit points equal to the amount of necrotic damage dealt to that creature.

https://www.5esrd.com/database/spell/life-siphon/

The table of 6 level 10 players are going up against a big bad undead boss. Felt like giving it this spell as a one-time use during the battle would up the stakes a bit. It feels a bit strong, so I might tweak the radius and maybe only half the hit points regained from the damage dealt. Thoughts?