The Hallowed Ground achievement is unlocked during Chapter 6, "We Don't Go to Ravenholm". After navigating through Ravenholm, you'll encounter Father Grigori, who will assist you through a cemetery filled with Headcrab Zombies. Successfully escorting him to the end of the cemetery will unlock the achievement. Father Grigori is armed and can defend himself, but staying vigilant and assisting in clearing the zombies will ensure a smoother passage.