Magnetar was a device developed by Mercury Labs. It could act as a pulsar – a power amplifier with a highly magnetized, dense rotating core that could be easily weaponized. Hunter Zolomon, a meta-human speedster from Earth Two, stole the device and turned it into a weapon that would allow him to destroy all of the Multiverse outside of Earth One with a trans-dimensional shockwave. The device was almost successful in this goal, if not for the sacrifice of Barry Allen's time remnant, who used his speedster abilities to create an out of phase pulse of his own, counteracting the pulsar, but sacrificing his own existence in process.