The Last of Erebus (known to Torchwood as Object One, the Bad Penny or the Silas Obelisk and to Jack Harkness as the Red Key) was a locket that had the power to select the worst possible future for those in its vicinity.
The object was given by the Committee to the Tsar Alexander of Russia in order to weaken the country with its power of bad luck, causing a rise in infant mortality amongst other things. The Tsar gave the object to Queen Victoria. Upon learning of the object's power, she gave it to Jack Harkness to throw into the Cardiff Space-Time Rift. Holding onto it for a while, he eventually did so in 1914, when the First World War started.
The little girl came into possession of it, and gave it to Alex Hopkins in 1999. (AUDIO: The Torchwood Archive) He opened it and saw the future, causing him to kill his team-mates to spare them before killing himself. (TV: Fragments, AUDIO: The Torchwood Archive)
Suzie Costello came across the object in a piece of machinery in a factory creating robotic duplicates. Jack had her call Yvonne Hartman and give it to her in order to get it out of Torchwood Three's hands. After the Battle of Canary Wharf, it was given to Ianto Jones to take to Torchwood Three, although the employee that gave it to him had been told by Yvonne to throw it into the void. (AUDIO: The Torchwood Archive)
The Committee once again came into possession of the object and gave it to Anna Volokova to give to Toshiko Sato. (AUDIO: Zone 10) She managed to reverse engineer it to give good luck, but felt that nobody should have that power and sent it into another dimension, removing her memories of it using retcon. (AUDIO: The Torchwood Archive)
The Committee used a hologram of Norton Folgate in order to manipulate Andy Davidson. Andy found the object in a crashed spaceship, and Norton showed it to the Committee and took it back to 1953 per their wishes. (AUDIO: Ghost Mission)
Norton and his lover Jeremiah Bash Henderson planned to use the object to revive the Committee after their defeat in their war against the Enemy. To do this, Jeremiah infiltrated the Torchwood Archive to discover how it could be done, before exploding the place. (AUDIO: The Torchwood Archive)