The Story of Locked Dragon in Jinghu Lake: a Thousand Years of Atonement for an Evil Dragon

This is an old legend with good foundation - there is an old dragon chained up at the bottom of Taihu Lake. Whenever there is a severe drought, the dragon's head emerges from the water and people burn incense to pray for rain. But have you ever thought about it? What laws of heaven did Long commit back then that warranted him being locked up? Who locked it? Has it regretted it in these thousand years?