Unlike the previous two quests in this quest chain, you must enter the Underworld normally - either pay 1Image:Platinum.png or use a Passage Scroll to the Underworld. Once there, you will see (among the normal enemies) some Skeletons of Dhuum roaming around. Wait in the starting area until one walks close enough that you can aggro it without aggroing any Bladed Aatxes, then pull it down the stairs to you. After damaging it down to ~25% health, double-click the Everlasting Mobstopper in your inventory, and it will turn into a Captured Skeleton. You can still capture a skeleton if you kill it.