Kadavan

Kadavan was an ethereal mercenary hired by the death knight, Ragnok Bloodreaver, to capture nether dragons for him. Kadavan developed a device to carry out this feat, which Ragnok used to enslave a number of nether dragons. When the blue dragon Tyrygosa resisted the device, Ragnok threatened to kill Kadavan for his failure until the ethereal reminded him that he only promised that the device would ensnare nether dragons, which it had. In response, Ragnok ordered Kadavan to improve the device so as to capture Tyrygosa as well. Kadavan readily complied.

During Ragnok's attempt to seize the Dark Portal, Tyrygosa captured Kadavan and demanded to know how to shut off the device and free the captive nether dragons. Kadavan eagerly agreed in exchange for his own life, but before he could explain how to disable it Jorad Mace destroyed the device. Kadavan explained that though the nether dragons were now free, the device's destruction would cause the collars affixed to the dragons to rapidly siphon away their energy and kill them unless they found a suitable source of power to sustain them until the collars burned out. Tyrygosa opened a portal to Coldarra and called the nether dragons to follow her to the Nexus; Kadavan, still held fast in her claws, went helplessly along. Now stranded on Azeroth, he journeyed out to find new customers.

Speculation
As the Ethereum were seen in Coldarra, Kadavan may have joined them.