A priest's vision comes from the Door of Truth
The ceremony involves opening a rift into the "Otherworld" and petitioning spirits described as 'little faces'. When Nagomi asks to 'see what lays beyond' the spirits laugh and taunt her. When the vision ends, Nagomi feels exhausted despite having stood still for the entire thing. Instead of taking body parts, the cost could have been her a year or so of her life span. Apparently, instead of dicking around with alchemists Truth-kun gets its kicks sending vague visions to priests.
