The Thermopylae was, purportedly, a vessel in the Systems Commonwealth High Guard in the Battle of Witchhead. It was the flagship of the Commonwealth armada that was to rendezvous in the Witchhead Nebula before their final push to the Nietzschean capital planet at the time, Fountainhead. However, they were ambushed by a Nietzschean fleet waiting in the nebula, and the Commonwealth fleet was destroyed before it could fully organize and depart. Allegedly the Thermopylae and the Nietzschean flagship, the Shining Path, were the only vessels still combat capable by the end of the battle. Its apparent commander, "Teddy Roosevelt", who was at the time was dying, rammed his ship into the Shining Path as a final act of defiance. ("Angel Dark, Demon Bright")
This information, however, comes only from a story told by Harper, which, based on the skepticism of Dylan Hunt and Tyr, is not that accurate.