Horcrux

In the novels, a horcrux anchors a person's survival to an object. Making one requires a killing. This journal will not linger on method. What the plot needs you to know is the result: an antagonist who is not finished when his body fails, and a hunt organized around destroying anchors.

The objects themselves are a mixed set of heirlooms, trophies, and hiding places. Their interest is structural. They turn the last books from a duel into a series of losses.

Whether that structure fully resolves is discussed, carefully, in the Margin. The existence of the device is settled in the text.