The false skeleton mage is an enemy encountered while fighting Lucien during the final days of the 2018 Hallowe'en event.
During the battle with Lucien, bone piles will appear and Skeleton warriors, Skeleton rangers as well as False skeleton mages will be summoned from them, prompting the following message in the chatbox:
Lucien raises skeletons from below! Try not to get caught by their explosions.
Regardless of their attack style, the skeletons will explode after five seconds. Damage can be prevented by avoiding close proximity to the skeletons.