Past
Looking back at his tragic life.
Little is known about the lives of the ancient ryuzoku. They were pacifists and lived peaceful lives for many years and chose to remain neutral in the Kouma War that took place between the mazoku and ryuzoku 1000 years before the series takes place. Although they had access to the powerful light weapon Galveria, they felt that using it in the battle would cause more chaos and chose to stay out of it. The golden ryuzoku did not like the ancients' neutrality and feared that the ancients would one day turn against them. To quell their fear, the goldens decided the only safe course of action would be to kill off the ancients before they could become their enemies.
Val Agares was but a child when the attack on the ancient ryuzoku began. We see a flashback of him as a child, crying as he sits on the ground, surrounded by the corpses of those he once loved. From that point on, he would become hunted for many years, spending his life on the run from those that wanted him dead simply because of his race.
How long Val managed to live like this is anyone's guess but eventually the golden ryuzoku caught up to him. Severely wounded, the goldens left him for dead in the desert, fleeing from the presence of one of the five great mazoku subordinates of the Dark Lord Shabranigdu: Chaos Dragon Garv. Assuming that Garv was there to finish him off, Val weakly attempted to transform into his full dragon form but collapsed from the strain. Garv looked at him in pity and told Val that they were in similar positions.
Garv explained that he too was on the run from his own kind and that it was a shame to meet Val under the same conditions. Garv offered Val a choice: death or revenge. Val pointed out he was close to death already but Garv had a way to offer him salvation: die and be resurrected as a mazoku. This way, Val would continue to survive and those that wanted him dead would be furious for it. Laughing at the concept, Val accepts Garv's offer and is resurrected as Valgarv, Garv's loyal retainer.
Whatever Valgarv did as Garv's retainer is a mystery but we do know that he delivered Garv's offer of death or revenge to others in similar circumstances. He meets Gravos, a lizard beastman who's been left for dead by his own kind, and Jiras, a fox beastman who's kind had been hunted down and murdered by humans. Both Gravos and Jiras choose revenge and become loyal followers of Valgarv, faithfully doing his bidding.
Eventually, Garv is destroyed by Hellmaster Phibrizo during the events of Slayers NEXT. Wherever Valgarv was during this time is unknown but when he's told the news of Garv's demise, he was devastated. Like everyone else in the world, he believed that it was actually Lina Inverse who killed Garv. Consumed by anger, Valgarv vowed to get revenge for his master's death.
Before he could set out on his mission of vengeance, Valgarv was prepositioned by an otherworlder named Armace. They made a deal: in exchange for helping Armace find five weapons of great power, Armace would aid Valgarv in his revenge.