New Magic Item: Arrow of Time (Revised)This very rare arrow acts as +1 to hit and damage. If the archer scores a hit with it, then all action in the prior round is retracted, and that round is played out over again from the start. (The archer's shot with the arrow of time is not changed, and he or she only gets one loopback.) Afterward, probabilities for critical hits or fumbles are increased by one pip (i.e., if normally 1 or 20, then become 1-2 or 19-20) for all parties in the rest of the encounter.
This arrow is usually kept to reverse some calamity to the archer's friends (such as a sudden death; although the person rescued never has any perception of the fact or that the arrow does anything at all). Rumors exist of more powerful arrows of time +2 or +3, which can loopback a number of rounds equal to the bonus, and increase criticals by a like amount.
I think that's a little "weirder", as befitting the name of the item, and it also better matches the perception of "less random in the past, more random in the future" which the phenomenon originally describes. Thanks to Confanity & Thiles Targon for challenging me on that!