To get Ash of War: Sacred Blade in Elden Ring you have to kill the Teardrop Scarab near the Spiritspring Jump north of the Third Church of Marika, northwest of the Sending Gate opened by D, Hunter of the Dead. It’s best to sneak up on Teardrop Scarabs and kill them with a single heavy hit, otherwise they’ll scurry away and leave you in pursuit.
Ash of War: Sacred Blade grants the Sacred Blade skill to all melee weapons (except whips, fists, and claws). Sacred Blade throws a sacred arc projectile, dealing an additional 85 Holy damage to the weapon over 20 seconds. Adding the Ash of War: Sacred Blade upgrade also gives your armament the Sacred affinity, which adds Holy damage and Faith scaling while also slightly reducing physical damage and some damage denials.
Related: Where to get the Sacred Scorpion Charm in Elden Ring
Ash of War: Sacred Blade is very useful against those who live in death, or “undead” as most players call them. It’s also great for a Templar or Paladin, or any character build that combines holy, faith-based powers with melee combat (e.g. a Prophet or Confessor that you want to take in a martial direction). It’s not the best choice in every situation, but there are certainly plenty of undead enemies in Elden Ring that are weak to Saint damage.
https://www.gamepur.com/guides/where-to-find-ash-of-war-sacred-blade-in-elden-ring Where to find Ash of War: Sacred Blade in Elden Ring?