From Pathfinder Lost Omens Knights of Lastwall, which isn't enabled.

Flashing Shield

FEAT 8

Activity free action

Prerequisites Lastwall Sentry Dedication; you worship a good-aligned deity

Trigger You use the Shield Block reaction, and the opponent that triggered the Shield Block is undead and adjacent to you.

Your shield flares with holy light as you block the attack, searing the undead with the power of your faith. The triggering undead takes vitality damage equal to half your level and becomes Off-Guard for 1 round.

Pathfinder Lost Omens Knights of LastwallOGL