His real name is Sukuna, a Dragon Lord of the sea. He has been waiting to seek revenge against Tomoe for 526 years and 28 days ago, at 2 o'clock 18 minutes and 4 seconds in the morning for ripping out his right eye and attacking the north gate of Ryuuguu Castle. He found Tomoe after he saved a girl from drowning in the sea. The Dragon Lord's eyes are known as the Longevity Elixir and are said to give great power to whoever drank them.

(Source: Wikipedia)