Sakasugi is a red dragon.
Or, as one would put it, was.
Being a red dragon—the greediest and most arrogant draconic species amongst all five shades of dragon, Sakasugi was basically the standard template of evil dragons depicted in knight novels.
It was obsessed with plundering the treasures of humans and every other sentient race, and would incinerate its victims and villages whenever it felt like it, oppressing weaker beings and enslaving them so that they would plunder even more treasure or anything that interest them.
Such rampant vice naturally drew discontent. When Sakasugi's havoc finally affected humans too severely—or as one would say, the needs of the gods whom the humans worshipped, they took action.