The Chapter Black tape is an artifact from the Spirit World which is the focal point of the Chapter Black Saga. Originally kept in the deepest bowels of Spirit World Records Department, it is a compilation lasting for thousands of hours, said to contain the atrocities that human-kind had committed over millennia of years. The tape is so legendary, it is known to demons such as Kurama and Hiei, so much so that Hiei himself sought to acquire the item. According to Koenma, the Chapter Black tape is part of a set which is never meant to seen alone. 
The tape's contents are so horrifying that it can turn humans against their own kind such as the case with Kiyoshi Mitarai, where his thoughts were so warped, that he considered himself to be the perpetrator of the atrocities found in the video.
It vanished from the Spirit World Records Department vault along with Shinobu Sensui after he ended his tenure as a Spirit World detective ten years before the events of the series.
After the events of the Chapter Black Saga, Hiei acquires the Chapter Black tape and after Kurama inquires what Hiei will do with the tape, he destroys it.