to pass away (used as a euphemism specifically for the death of emperors and kings)

当那位统治了半个世纪的皇帝乘龙而去，全城百姓都感到悲痛。

to ride on a dragon (often associated with ascending to heaven and becoming an immortal)

传说黄帝最后乘龙升天，成为了神仙。