This shield is the highest result of the monster mission "Out with the Old" which is started by giving a Bristlebane's Ticket of Admission to a silly puppet in Kithicor Forest. The ticket was gained by completing various Halloween tasks.
This dropped last night on the third of three waves from the The Legion Commander of the Void. On the third wave he sat on his horse, and sent three additoinal waves of skels against us (each with their own leader.) When he fell he had six Shards of Wisdom and this. The Cdr was white to all of us in the Monster Mission (lvl 30.) He also summons.
According to a post of the quest walkthru for the Bone Mask of Horror, this item is dropped while doing the Bone Mask of Horror quest, by the last named mob, during the very last misson (the one where you defend Rivervale).