This spell can be cast as an Action or Reaction:

You may cast this spell as a reaction. When a creature you can see within 30 ft dies, you may use your reaction to absorb the creature's essence.

You may also cast this spell as an action. You raise a gemstone in one hand toward a creature or object in your possession you can see within range, drawing arcane symbols in the air with the other hand, and speak the incantation. While in combat, the target of the spell must make a constitution saving through against the caster's spell save DC. Objects immediately fail this saving through. A creature takes 10d12 necrotic damage on a failed save, and half as much on a successful one. The caster gains half of the damage taken as temporary hitpoints. A creature whose CR is greater than 3 times the spell slot level used to cast the spell immediately succeeds on this saving throw. If a creature is killed in this way, their essence is absorbed by the caster and the caster gains some of the benefits as stated below. A creature or object killed in this way is immediately destroyed and can only be restored by a wish spell. A creature that is magically imprisoned, such by an imprisonment spell or spell with similar effects automatically fails this saving throw and is immediately absorbed by the caster, ignoring the CR of said creature. A willing creature not under the effect of any kind of enchantment magic may willingly fail this saving throw and be absorbed, no matter the CR of said creature.

Absorbing a magical object gains you the following benefits based on the rarity of said item or CR of said Creature:

• Common or under CR 8: The wearer gains 5 temporary hit points.
• Uncommon or CR 8 to CR 12: The wearer gains 10 temporary hit points.
• Rare or CR 12 to CR 16: The wearer's maximum hit points permanently increase by 2. They also gain advantage on attack rolls and ability checks until the next long rest.
• Very rare or CR 16 to CR 20: The wearer's maximum hit points permanently increase by 5. They also gain advantage on attack rolls, ability checks, and saving throws until the next long rest.
• Legendary or CR 20 and above: One of the wearer's ability scores increases by 2, as well as their maximum for that score. Particularly powerful items and creatures can bestow additional benefits* to the caster.

*Legendary Creatures always provide at least one minor additional benefit