- "This Magnetar that's being developed by Mercury Labs... Turns out, it can act as a pulsar."
- —Harry Wells on Magnetar[src]
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.
The original Magnetar that was stolen by Hunter Zolomon was a much smaller device, with a sphere at its center. Hunter tinkered with the device, in an attempt to weaponize it.
After being changed by Hunter Zolomon, Magnetar was expanded into a gigantic "race lap" that would allow it to be powered by the Speed Force energy produced by speedster meta-humans. According to Hunter, one lap around the inside of this loop would produce about 1 gigawatt of energy, and 500-plus gigawatts were needed to power the Magnetar enough to send a trans-dimensional shockwave that would destroy alternative Earths and their timelines. However, the pulse from Magnetar could have been counteracted by another, out of phase pulse. 
At some point of time, Magnetar was developed by Mercury Labs for a scientific purpose. However, Hunter Zolomon, along with Harry Wells discovered an alternative purpose for it. As Harry explained, the Magnetar could be used as a pulsar, a power amplifier with a highly magnetized, dense rotating core that could be easily weaponized. Hunter stole the device and tinkered with it, turning it into the weapon to destroy all alternate Earths outside of Earth One. 
While Hunter was tinkering with the device, he was approached by Caitlin Snow's holographic image, who distracted him and allowed Team Flash to weaken the speedster, banishing him back to Earth Two. But their plane backfired, Zoom taking Joe West with him as he was flung through a breach, ending up a prisoner back at his lair.
Hunter later "raced" against Barry Allen in the inner laps of the device, powering the device together, and threatening to kill Joe if Barry refuses to race. Although his plan was partially successful, Barry "stole" Zolomon's trick with time remnants, allowing different versions of him to both overwhelm Zoom and sacrifice themselves to create their own pulse that would be out of phase, counteracting the one from Magnetar. This led to a time remnant's death and Barry's triumph over Zoom. 
- 500 gigawatts needed for the weaponized version of Magnetar is about half the total energy production capacity of the United States of America.
Behind the scenes
- The debacle of achieving enough gigawatts of energy to power a science device was a plot point in another time-travel related series, "Back to the Future".
- The tachyon device of Mercury Labs that appeared in "The Reverse-Flash Returns" physically resembles the version of Magnetar before changes brought by Hunter Zolomon. However, it wasn't explicitly confirmed to be the same exact device.