In the very beginning, Shashi was only a miko for Ashura-ou (along with her sister Kara), but well, it seems like being a miko for the great warlord wasn't good enough. So later she got married to Ashura-ou and became his wife. But it still wasn't good enough for her, so during the Holy War, she helped Taishakuten in his rebellion by telling Taishakuten all the secrets of the Ashura clan so he could win. She betrayed Ashura-ou and his whole clan, which was why they all got wiped out. She became Taishakuten's wife, "Empress" Shashi. Shashi gave birth to twins, one being Taishakuten's son and the other being a child of the Ashura clan (Ashura-ou's child). So she tries to kill her own child (Ashura) with her own hands.

(source: Nishi's RG Veda Gallery)