Defeat Skull Spearmen, SkullTroopers, and Skull Archers in the Defiled Pasture to collect Skull Soul Pieces. Then, talk to to Farmer Keifer at Stratus Farm. Use your quest tracker to see your progress.
Farmer Keifer: You've slowed it down - that's good. Headquarters also Farmer Keifer: seemed relieved to hear you're on the case. NakanoAzusa: I don't know what's causing skeletons to spread. There NakanoAzusa: was a similar incident at the Great Hessian Tree before. Farmer Keifer: Headquarters gave you an order to harvest soul pieces of Farmer Keifer: skeleton and bring them with you to the headquarters. HiiragiKagami: They must be preparing to really delve into the incident. NakanoAzusa: Got it. I would like to ask you to work as a messenger NakanoAzusa: between the headquarters and I for a while. Farmer Keifer: Sure. I've been a farmer all my life, but now I feel Farmer Keifer: like a soldier. I'll do my best.
Reminder Dialog
Farmer Keifer: I didn't know skeletons still had souls.