Ancient dragons are the most powerful of the ryūzoku but were hunted and killed almost to extinction by the golden dragons due to their fear over the ancient's power. It is at one point claimed in Slayers Try that each ancient dragon was as powerful as thousands of golden dragons, but considering their ultimate fate, this seems to be somewhat of an overestimation. Little else about the ancient dragons is known other than that they were a pacifistic race; this quality was one of the factors that led to their demise.
During the Kōma War, the ancient dragons refused to side with either the get involved, fearing that the use of the weapon Galveira, which they possessed at that time, would bring even greater chaos to the world. Despite this pacifistic stance, the golden dragons still feared that the ancient dragons would use their power, as well as Galveira, against them. To prevent this, the golden dragons slaughtered the ancient dragons.
To current knowledge, Valgarv, originally named "Val", is the last known ancient dragon. He became servant to Chaos Dragon Garv when the rogue mazoku lord saved the close-to-dying young dragon. Valgarv was killed at the end of the series Slayers Try, but was reborn as a dragon and put into the care of Filia Ul Copt; thus, he is still the last known ancient dragon in existence.
Ancient dragons are black in color, noted for having large, bird-like wings that look like they are covered in jagged feathers (though they may be scales), rather than the usual leathery ones typical of most dragons in the Slayers world.