From Diablo Wiki
A Funerary Urn in Diablo III is a special Event-related object that a player must interact with.
The urns are used for an event called the Matriarch's Bones, which has a chance to occur in the Cathedral shortly before the player encounters Leoric. There are three urns that must be clicked on during this event, each urn spawning a guardian champion that the player must defeat.
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