The city sits over a large fault line that runs across the former kingdom and into what was the western part of Valdemar at that time. A hole was punched in the fault during the Mage Wars, leading to extreme instability in the fault. The Tayledras, who once lived where Highjorune is now situated, left an active Heartstone in place, anchoring a spell that not only stabilizes the fault, but is slowly repairing the mage-wrought damage.
At the time of the annexation, Herald-Mage Vanyel fought the mage Vedric Mavelan, and the rest of the Mavelin family, over the fate of Lineas. He pulled enough power from the Highjorune node to destabilize the fault. The resulting earthquake toppled the royal palace, burying the Heartstone for good.