To unlock this achievement, you’ll need to equip a sickle in both hands. You can find one, the Bone Sickle, by completing the Black Stout mystery in Connacht. Another, the Ceremonial Sickle, can be looted from Lackanscaul, which is a Druid camp west of Clonfert Abbey.

Once you have both sickles, all you need to do is equip one in each hand so that you’re dual-wielding them. The achievement should then unlock, however you may need to reload the game if it doesn’t unlock immediately.