Long before skulls became shorthand for rebellion, fashion, or Halloween décor, they served a much simpler purpose: a warning. A reminder. A confrontation.

This month’s From the Vaults of the Unhinged feature is Memento Mori: A Skeleton in a Niche, a late 15th-century engraving by Master IAM of Zwolle, a Netherlandish printmaker active around 1470–1495. Now held in the collection of the The Metropolitan Museum of Art, the engraving is modest in size—just over 6 inches tall—but its message is anything but subtle.