This short side-quest can be found in the Cemetery where we fought the Worshipper earlier. Clear the area of enemies and once it is safe, look for a hole in the ground beneath a large tree (almost directly opposite the gate to the demon realm portal). You can interact with the hole.
If you have the Cursed Sword (which we can find in the Old Quarters section of Keystone Citadel) you will gain the option to bury it. Doing so will summon a spirit which will attack you and you’ll need to dispatch it.
Killing the spirit will earn Harkyn 1,500XP , an upgraded version of the Sword (Sword - Cursed Sword) and an achievement/trophy 'I Feel Lucky' .