Ambassador Kosh arrived in 2257 to represent the mysterious Vorlon race on Babylon 5. Their dealings with Earth, and all races for that matter, have been quite limited, they themselves claiming that they "take no interest in the affairs of others." In late 2260, Kosh was killed inside his quarters; no one knows exactly what happened. A replacement Ambassador was sent, doning a different encounter suit. However, his actions are consistent with everything that is to be expected from a Vorlon.
After the Vorlons began systematically destroying planets that had been touched by the Shadows, it seemed time for the Alliance to finally move against them. After luring him into the trap and cracking his encounter suit, Captain Sheridan released some of his life energy along with what Kosh had left inside him and struck down the Vorlon. Now that all Vorlons have left this galaxy, it is unlikely we will ever see another Vorlon again.