When a large star dies, sometimes it becomes a neutron star, a tiny, 12 mile across ball that's made almost entirely out of neutrons. These dead stars are incredibly dense, and spin incredibly fast. Just one thimbleful of neutron star would weigh 100 million tons. Magnetars are a variation of neutron stars, and they somehow manage to be even scarier. Neutron stars already have extremely strong magnetic fields--about 2 trillion times more powerful than Earth's. Yet magnetars have magnetic fields 1,000 times stronger than that. Yeah, that's a pretty intense field. Magnetars are not just insanely powerful--they're also very, very dangerous. If you were a mere 1,000 kilometers from a magnetar, your entire body would dissolve as the magnetic field rearranged the sequence of atoms in your body.
But don't worry--thankfully, there aren't any magnetars near Earth. The closest one is about 9,000 light years away. Let's pray that it stays that way.
In addition to their terrifying magnetic powers, magnetars also have something called starquakes, which function similarly to earthquakes here on Earth--except with much more intense results. A crack in the crust of a magnetar is responsible for the brightest light we've ever observed from space. And if a magnetar was closer to us, like 10 light years away, and blasted us with the radiation from a starquake, it would destroy our ozone layer and probably kill all life on Earth.