ASASSN-15lh was an extremely bright object, probably a Hypernova event located in a faint galaxy 3.82 billion light years from Earth, and shone more than 570 billion suns, brighter than 20 galaxies like the Milky Way! It be exceeded by the new brightest one, the PS1-10adi, and was about twice the luminosity of the iPTF13ajg.
The nature of what caused this explosion is unknown to Earth, but most likely it was the brightest Hypernova ever seen, or perhaps a Tidal Disruption Event caused by a supermassive black hole, and some other hypotheses are gravitational lenses, the Quark Nova or a Magnetar spindown
It was the main hypothesis created by Earth for this event, also called Hypernova, but it has many peculiarities, like a very massive star, like a Wolf-Rayet inside a star that produces few stars, and mainly the lack of hydrogen and helium.
It was probably a Supernova caused by the rotational energy of the hot material around a rapidly rotating neutron star, but it is impossible because the explosion's luminosity was exceedingly above the limit.
This explosion may have been a Quark Nove inside a Supernova of a Wolf-Rayet, Quark Nova happens a few days after the neutron star has formed as a remnant and this may create the peculiarities seen in this Supernova, but this is unlikely to be that.
One of the known methods to generate a huge amount of energy is the disruption of a star passing near a supermassive black hole, and that Hypernova happened near the nucleus of the galaxy, where the supermassive black hole must be located.
However, even the mass of the supermassive black hole would not be enough to destroy a star with this violence, however, a supermassive black hole spinning rapidly can destroy a star with the mass of the Sun still forming the Event Horizon and produces a disk of and an absurd luminosity, but the lack of helium and hydrogen is a big problem in this theory.
Possibly, a peculiar explanation but simple is the solution, a Supernova inside a gravitational lens can bring the luminosity of great and massive objects to nearer, however, there are no gravitational lenses between the galaxy and the Earth.