The Chain of Memories is a special item that was deemed so dangerous by the captain commander in an era around the beginning of the Soul Reapers that it was sealed away in the Dangai Precipice World.
Legends says that the Chain of Memories was created by a sword creator, who used his craft to make blades for the Soul Reapers as at the time how to unlock one's Zanpakuto was unknown and thus happened rarely.
While travelling in the woods the crafter was almost struck by a small meteorite. The object he found at the crater was a metal unlike any he had ever seen before. There was only a small amount of it, but once he melted the metal down, he saw some of his memories clearly on it's surface.
Deciding to craft a new object using this metal that would be incrediblly valuable, the craftsman spent day and night for a week forging the object. When he was finished, he foudn himself with a small chain of crystal shaped objects that were floating together connect by absorbing reiatsu from the air.
The craftsman brought the Chain of Memories to the captain commander. The commander used it in battle, and the chain, having absorbed the hollows memories, brought forth it's enemies from the past, which tormented the hollow to a large degree as the person the chain brought forth was much stronger than before.
The Commander decided the chain was too powerful and tried to destoy it. Nothing worked and thus the chain was stored in the Dangai Precipice World, where it has lain ever since.
Memory Creation: By absorbing one's memories, the chain's wielder can call forth people, places, and things that were absorbed. They are not truly the actual thing but they are exact copies, and the people even have the same memories, but are also informed of the times and are under the command of the person using the chain.