Collect Undead Skeletons is a quest available after Edena Village has been passed for the first time. It is part of the main quest line and becomes unavailable after it has been completed.
To complete the quest, 40 Undead need to be killed in Edena Village. For each Undead killed one Undead Skeleton is collected, that counts towards the quest goal. At least two dungeon runs are needed to achieve this.
Click on [Campaign] -> Select the [Edena Village] -> Kill 40 the Undead to collect [Undead Skeletons]x40 The Undead have been keeping deceased bodies for themselves. Retrieve them so their soul may find rest.