This entity originated in Sector 14, near the area of the temporal rift. It has the ability to live inside the mind of a sentient being, similar to a Vorlon. It first took over the mind of Chay-yosh-nay, a Markab trader on his way to Babylon 5. The trader received hallucinations, images sent by the entity in an attempt to communicate with him. Eventually, these images drove Chay-yosh-nay mad, and he killed himself by bashing his skull against a pipe in Downbelow.[1]
Eventually, Captain Sheridan found Chay-yosh-nay's body while searching the area known as the "Babylon Triangle", after hearing enticing stories from Michael Garibaldi. Shortly thereafter, he too began experiencing hallucinations. First, he was attacked by a grylor, an animal native to Janos VII that once stalked him. Next, he saw his wife's ship, the Icarus, explode in space from C'n'C. Finally, while in his quarters under medical observation, he saw an image of his father and mother, holding their arms out to him as though welcoming him home.
He understood the images he had been receiving; the entity was using elements of his own life to communicate with him. Each image had a specific meaning to Sheridan: "fear, loss, home." Sheridan escaped from the station, taking a Starfury out to Sector 14, where the entity forcibly left his body and went back through the temporal rift.
While discussing the events with the rest of the command staff, Dr. Franklin theorized that the alien lifeform was conscious and aware, yet totally outside their realm of experience. It was likely an energy lifeform, and placing those images in the captain's mind were the only way it could communicate with him. However, they were unable to verify any of these theories, as Sheridan was reluctant to ever go back and try to make contact with the creature again.