That which Moraelyn and Edward between them used to free High Rock from Nord domination of their spirit. Properly used, none can stand against it, not e'en that power which the Emperor controls.
He knew the Staff lay there, not the Horn, and that he must teleport to safety, for the Staff is not a weapon that would have seen him clear, unlike the horn. Praise the gods he hath not that!
Tiber Septim claimed the Horn, but left the Staff for safe-keeping. Nightingale can use the Staff to sow seeds of strife and dissension, if he wishes, yet that alone will not gain him power. That lies with the Horn and the ability to use it.
So this implies that it's extremely powerful, even more-so than the Staff of Chaos which is a very potent artifact. And it requires some special ability to utilize.
It also has a link with Moraelyn and Edward, whose story can be found in http://www.imperial-library.info/content/daggerfall-king-edward, although that's extremely out-of-date.
This probably doesn't help you much, but it's all I know.