The Mournful Cry in AC Valhalla is one of the best hammers in the game. Most hammers in the game have a very high damage output. What makes the Mournful Cry so powerful is its side effect. Mournful Cry can poison opponents upon landing a critical hit. Landing a critical hit isn't always easy, but the chance at poison damage gives players with this hammer the edge.
Mournful Cry AC Valhalla: How to Get the Hammer
The only way to obtain the Mournful Cry hammer is to purchase it from Reda at his shop in Ravensthorpe. The hammer costs roughly 51 opal. Unfortunately, the item isn't always for sale. Reda's Shop switches out its items every day. This means that if the item isn't there when you check the shop, you'll have to come back the next day to check.
Everything considered, the Mournful Cry is well worth the patience and the price.