Defeat Undead 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. At least two dungeon runs are needed to achieve this.
Click on [Campaign] -> Select the [Edena Village] -> Kill 40 [Undead] The residents and refugees of Edena Village have been killed. Eliminate the Undeads, in memory of the deceased.