The Cei were an aquatic race. They had tentacles and were a large, squid-like alien. They found Oxygen poisonous, with their tentacles turning purple if they came in contact with an atmosphere containing it. They preferred to breath Nitrous Oxide.
During a war between the Cei and another planet, the Cei were defeated but sent a Terraforming Device to Earth. The device, which took the appearance of a large gold sphere, was intended to turn Earth into an aquatic world and an outpost for the Cei. (AUDIO: The Runaway Train)