Amphibious. The dragon can breathe both air and water.
Spellcasting (Psionics). The dragon casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 12):
1/day each: bane
Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit:7 (1d10 + 2) piercing damage plus 2 (1d4) necrotic damage.
Desiccating Breath (Recharge 5–6). The dragon exhales yellowish necrotic energy in a 15-foot cone. Each creature in that area must make a DC 11 Constitution saving throw. On a failed save, the creature takes 21 (6d6) necrotic damage and is weakened until the end of its next turn. A weakened creature has disadvantage on Strength-based ability checks and Strength saving throws, and the creature's weapon attacks that rely on Strength deal half damage. On a successful save, the creature takes half as much damage and isn't weakened.
Decay and despair are bound up in the nature of topaz dragons, thanks to the necrotic energy of the Negative Plane that suffuses them. Their psionic power manifests the fundamental entropic principle that mortal beings and their creations are ultimately doomed to death and decomposition, and the dragons' demeanor is typically morbid and curmudgeonly as a result.