This quest appears only in Enhanced Edition of Divinity: Original Sin. In Hunter's Edge you will find a woman named Cordelia who is an immaculate necromancer. She found a way to reanimate skeletons back to life. The two merchants, Hortun and Charla, whom you should have met before (Hortun in front of Hunter's Edge and Charla on Cyseal's Beach) are her work.