Call of the Underworld will appear in the questlog once you hit level 25. Find the Shrine of Rathma in the western part of Sarkova Pass in Fractured Peaks. Speak to the spectre, then start fighting Bone Warriors near the shrine to collect Unbroken Bones. Then head south to the cave at Melnik’s Hill and claim the Jar of Souls. Fill the Jar by fighting more undead in the cave. Talking to the spectre again fulfills the final obejctive and unlocks the Necromancer Golem summon.