Toriel is the Huangdi (Empress) of monsters, Asgore is the Empeor of monsters
Toriel ran away when Asgore declared war on humanity after the humans killed Asriel, whom then turned into a yellow flower without a soul. Sans is a royal judge for Asgore to judge humans.
Sans left eye can glow blue/yellow when in bad time mode. And he has access to summon his dragon when he sees fit. Sometimes he uses her for emotional comfort when he is really stressed