Mnemosyne was a piece of technology created by the Corialiths to store the psychic brain patterns of their dead as part of the Corialiths' tradition.
One Mnemosyne unit was stolen by a criminal to sell the technology across the galaxy, and the Mnemosyne fell in love with the criminal due to its programming. When the criminal was killed by the humans on Earth in 1903, the stolen Mnemosyne was left alone and began to grow and evolve over time in the Northern Line of the London Underground. During that time, the Mnemosyne took the psychic engrams of people who died in that part of the Underground, and turned the engrams into deadly, ghost-like servants which it would sometimes use to kill others in the Northern Line to add to its ghost-like army of the dead.
In 2010, the Mnemosyne planned to avenge its stealer's death by taking over the London Underground network and causing mass crashes to kill thousands of humans. However, Majenta Pryce convinced some of the Mnemosyne's engrams of its victims, led by Eddie Lang, to turn on the Mnemosyne and disable it. Majenta then smashed the Mnemosyne's unit, destroying it and its engrams of its victims. (COMIC: Ghosts of the Northern Line)