Bitter Blood

The third and last creature on the list is a giant spider that inhabits the halls of Neborhest's Tower. The Oathbearers have seen the spiders, but avoid engaging with them, lest they get caught in their webs. This will be no easy task, and you'll have to brave the webs. But first, you must scale the hill to Neborhest's Tower.

Travel to Neborhest's tower. Kill the Soulless Drinkers within until you kill one gently enough to not rupture its blood sac. Once you have all three samples, return to Ralla Widmer in Oathhold. She will no doubt be grateful for your service.