Once upon a time; when the four death gods Hakai, Rekishi, Genrai and Mirai had gotten banned from the shinigami realm, whilst the king crowning them as lethally harmful for his kingdom; they had no idea what was the most dangerous thing that would approach to them -boredom. They got to keep their notes afterwards of their banishment, but life was too long for a death god, suicide was a sore escape, and boredom was the enemy. They gave up their death notes to the mortal world eventually, after compromising.