Urgo-Hem was freed from imprisonment through the blood of a 16th century Spanish priest. The priest had spent time in Tenochtitlan before it burned, and had learned the history of the Ogdru Hem there. After being persecuted by the inquisition for his knowledge, the priest was eventually squirrelled away by a secret order dedicated to preserving treasures of the ancient world.
When Hellboy stumbled across him, the priest's blood released Urgo-Hem. Apparently an aquatic beast, this child of the Ogdru Jahad resembled an eyeless serpent with sharp crustacean legs lining its pale underbelly. After a bloody conflict, Urgo Hem impaled itself on the mast of a nearby shipwreck, and Hellboy escaped.
Later, when Rasputin was brought back to the mortal plane and revived all of Earth's dormant Ogdru Hem, Urgo-Hem was seen towering over Amalfi, Italy in a much larger (but still recognizable) form.