pf2e.fyi
From Pathfinder Impossible Magic, which isn't enabled.

Void Siphon (Necromancer)

FEAT 4
CONCENTRATENECROMANCEROCCULT
Activity reaction
Trigger You would take void damage.
Requirements You are living.

You open your body to absorb void energy. You gain resistance equal to your level to the triggering void damage. After you take the triggering damage, you gain temporary Hit Points equal to your level. These temporary Hit Points last for 1 round.

Pathfinder Impossible MagicORC