Ashura is the heir of the Ashura tribe; a tribe declared to be the strongest guardian warriors; the gods of war. He is also the last surviving member of the tribe. Even before his birth it was prophesied that Ashura would be the one to bring forth the destruction of the world; the schism that splits the heavens. But even so his father wanted to give him the chance to be born and live. Because of his true nature Ashura has two personalities. One is of the innocent child that cares for everyone around him; the other is of the god of destruction that only wishes to be surrounded by death.
Yasha is the leader of the Yasha tribe; a tribe of strong warriors second only to the Ashura tribe. Yasha is a reluctant follower of Taishakuten, doing so only for the welfare of his people. Yasha is extremely devoted to his believes, and to whom and what he decides to cherish most. In his own words he would sacrifice everything to protect that one thing.
RG Veda starts with a fallen kingdom ruled by the tyrant Taishakuten. Yasha and his tribe are the reluctant protectors of the king since the Ashura tribe was extinguished 300 years before. Intrigued by a prophecy by the priestess Kuyou, Yasha discovers Ashura; the last surviving member of the Ashura tribe and according to Kuyou the one destined to kill Yasha. But instead of being wary of such a prophecy Yasha pays no heed to her words and immediately gets attached to the child.
Yasha ignores warnings from the elders of his tribe about Ashura and eventually the Yasha tribe is exterminated by Taikashuten's army with only Yasha surviving. He is now a king with no people to lead and with only Ashura beside him he decides to follow the prophecy and search for the six stars, because it is after this event that Yasha believes the prophecy dictates that it is their destiny to bring Taikashuten's tyrannous rule to an end.
With time the sixth starts gather: Ashura, Yasha, Souma, Ryuu, Karura, and on Taikashuten's side: Kendappa. Followed by Kujaku the star gazer they storm the castle where Ashura's sword; the Shura sword; seal is broken and with it the real Ashura is awaken; the one who wishes only for death and destruction. One by one the stars die until only Yasha remains; the last seal. Because the prophecy wasn't about the sixth stars reclaiming the kingdom from Taikashuten, but a warning that Ashura will be the one to destroy the world. Each star is a seal to prevent that, as long as they live Ashura will not fully awaken to his full power.
Taikashuten's rule and his tyranny were all a means to an end. A promise he made to Ashura's father; Ashura-Ou; the one he loved more than anything. A promise that he would not let Ashura become the God of Destruction. Yasha refuses to believe that the Ashura he knew; that his Ashura would forget their time together; their promise to live for each other. Even after Ashura confesses to Ryuu's murder, even after stabbing Yasha and telling him that his Ashura is dead. To Yasha, if his Ashura is dead then he has nothing to live for.
The real Ashura awakens with the power of the five fallen stars, ready to take the last one he lunges towards Yasha sword raise to his heart. Yasha doesn't try to stop him and closes his eyes as he recalls their promise. Blood splatters across the floor and Yasha opens his eyes to see that Ashura has stabbed himself through the heart in the last second. The prophecy has broken by Ashura's own will, taking his own life instead of the one he loves.
Ashura's body goes into a deep sleep that he will never awaken from. Peace finally settles on Tenkai and years pass. Yasha never leaves Ashura's side, waiting for him to awaken. Seeing the man so determined Kujaku gives his life and blood to fulfill Yasha's wish and awaken Ashura. The boy is tortured with the knowledge that everyone died because of him and Yasha just tells him he needs him, that he can only live on because Ashura's with him, asks him to be by his side for the rest of their lives.
That was their promise