In life, Marley, like Scrooge, was a bitter, greedy and selfish man. When he died, he was damned to eternally wander the earth as a decrepit spirit, forever burdened by a mass of chains that represent his accumulated sins. On the 7th anniversary of his death, which falls on Christmas Eve, Jacob Marley's ghost visits Scrooge in his house, warning him that he will suffer the same fate if he does not change his ways and informing him that he will be visited by three spirits later that night. Marley was presumably made visible to Scrooge for this visit as part of the arrangement among the spirits to convince Scrooge to change his ways. Marley warns Scrooge to expect the first ghost when the clock tower strikes 1, the second the same night at the same hour, and the third upon the same night when the clock tower strikes 12. Because A Christmas Carol is a story involving time travel, all this could happen in one night.
Scrooge is initially against this idea and unsuccessfully suggests that the three ghosts visit him all at once to get it over with faster as Jacob Marley leaves the room and disappears. When Scrooge is dragged towards the window, thanks to one of Marley's chains, he is horrified to see thousands of spirits who, like Jacob Marley, were all green, misty ghosts bound in chains.