Onyx dragons inhabit dismal swamps and dark fens. They are not especially intelligent dragons, but are cunning and stealthy when hunting prey.

At a distance of 120 feet or more, the onyx dragon cannot be distinguished from a black dragon, unless there is bright sunlight (in which case the dragon’s bright scales are conspicious). At closer range the onyx dragon’s lustrous, jeweled scales shimmer and sparkle in the light, revealing the mighty reptile’s true nature. In sunlight, the onyx dragon’s deep black scales shine as brightly as a thousand mirrors.

Onyx dragons speak their own language and the tongue known by all gem dragons. They also know a smattering of common (or perhaps another humanoid tongue spoken by creatures nearby).

Combat: The onyx dragon’s breath weapon is a cloud of inky black acid, all foulness and corruption.It has two effects. The first, identical to the black dragon’s breath, is a stream of deadly, corrosive acid (half damage if a saving throw vs. breath weapon is successful). The second effect comes into play only if the first saving throw is missed: the victim must make a second saving throw – this time vs. spell – or a darkness spell effect (15-foot radius) appears, centered on the victim. It moves as he moves. It can be countered by a light spell, or removed by dispel magic (use the dragon's age category as the opposing caster level); otherwise, it remains for 1 round per Hit Die of the dragon. This darkness is a special type through which the dragon can easily see; it otherwise functions exactly as a normal darkness spell.

If the second saving throw is successful, the darkness remains, but is not tied to the target; the victim can move and escape it.

Onyx dragons sometimes use the darkness breath in advance of an encounter, to hide themselves and gain surprise.

Habitat/Society: Onyx dragons lair in wetlands, ranging from the tropics to the chillier fringes of the temperature zones. As a rule, they prefer dark, well-shaded areas such as tree-filled bogs. In darker areas, their scintillating scales are less likely to give away their location to prey.

Ecology: Onyx dragons hunt the largest animals in their environments: alligators, herons, manatees, moose, and so forth. They do not hesitate to consume humanoid prey, but they do have enough intelligence to realize that adventurers may be more trouble to kill than the meal is worth.

Onyx dragons do value treasure, especially dark and lustrous gems. However, they tend to have less treasure than other dragons.