These daggers are extremely rare, or maybe there is only one... In any case, they work with experimental temporal magics. When thrown, the chronomantic magic within the blade activates and essentially pulls a copy of itself from a number of "temporal possibilities" where it was thrown. The temporal loop is not picky however, nor can it be, and can pull copies that were destined to miss.
The Dagger of Many Daggers is a +2 magical dagger. The Temporal Loop activates when the dagger is thrown. The item has one charge, and when it is used to attack, you may expend one of the charges. If you do, the dagger functions as normal, but it begins to multiply mid-attack. Roll 2d4, you gain that many extra attacks with the dagger, against the chosen target. This item regains its charge after one hour of use.