Highschool DxD is a Japanese Light Novel series serialized by Fujimi Shobo's Dragon Magazine, created by Ichei Ishibumi, and illustrated by Miyama Zero. The first volume was released on September 20, 2008 and has now has continued on to have a total of 23 volumes as of March 18, 2017
The story revolves around a human named Issei Hyoudou, who turns into a devil after being killed by his first girlfriend, Yuuma; he is then reincarnated into a devil by Rias Gremory. Issei now meeting a whole new world which was always thought to be stories and myths, strives to make his own harem and become the "Harem King", while also fighting demons, dragons, and even Gods.
A manga adaption illustrated by Hiroji Mishima, has also been released by Dragon Magazine. It was then moved to Monthly Dragon Age and has released seven volumes as of December 9th, 2014. There has also been two spin off series, "High School DxD: Asia and Koneko's Secret Contracts!?", which follows Asia doing her first set of jobs with the help of Koneko; while the second one being High School DxD: The Work of a Devil, which simply consists of the short stories that were already seen in the Light Novels.
An anime adaption has also been announced as of January 6th 2012. Following that, a second season which aired on July 7th, 2013 has also aired. On April 3rd, 2015, the first season was announced. The anime is produced by TNK and has aired on the Japanese channel At-X. As of now, there has been 36 episodes along with 16 OVA produced by TNK. The Anime has so far covered all of volumes 1 to 6 with some things from volume 7 included.