Events Library

The Events Library was an archive of whole events, heavily implied to have been created by Rassilon. It had several secret locations throughout the universe, each one archiving events pertinent to that region of space. Each "branch" had the goal of collecting data on the happenings of that part of space. Events could be stored at a branch in their entireties, as they occurred. When necessary, events could be "re-combined" to create a new reality which the proprietors of the Library found created a more desirable outcome of events. Thus, it acted as a "governor" on the pace and shape of events, allowing its librarians to contain conflicts.

It was thus a "two-way" archive, used not only to catalogue events, but also to rewrite history.

Nevertheless the Library had potentially malicious power in that it could completely erase individuals from time — including the Doctor. (DWM: Catalogue of Events)
 * The Library may have used technology similar to that undergirding the Matrix.