tanaquil:

THE TIME TURNER was a device capable of time travel that resembled an hourglass on a necklace. The number of times one turns the hourglass corresponds to the number of hours one travels back in time.

Time-related magic is unstable, and serious breaches in the laws of time result in catastrophic events. Possible scenarios include a wizard or witch killing their past or future selves by mistake, or altering one’s life path in such a drastic fashion that it can result in temporal anomalies such as un-births.

(requested by threemarauders and tessarion)