Styx dragon

Styx dragons, also known as darkwyrms or shadowdrakes, were dragons that dwelt in the corrupted depths of the River Styx and the River of Blood.

Description
The bodies of Styx dragons were long and serpentine. Their slimy scales ranged in color from dark brown to rusty red and their eyes glowed with a sickly yellow light.

Styx dragons were completely adapted for life underwater. Although capable of living indefinitely on dry land, they were incapable of flight and their short webbed limbs were ineffective for movement on the ground. However, they excelled at swimming and underwater combat and hunting.

Styx dragons were completely immune to the deleterious effects of the River Styx. Their diets consisted mostly of fiends and creatures they could catch, but they tended to prefer carrion.

Combat
Due to their atrophied limbs, Styx dragons could not effectively attack with their claws. Instead, they had adapted their long bifurcated bladed tails to be devastating cutting and constricting weapons. Both their tails and their bites infected their targets with a disease known as Stygian wasting, which caused flesh to rot away and hair to fall out over a period of several days.

They possessed two breath weapons: a cone of acid that had a slow, lingering effect, and a cone of gas that drained the intelligence of any creature hit by it. They could also innately cause effects similar to the control water, curse water, deeper darkness, feeblemind, fog cloud, hold monster, horrid wilting, mind fog, and stinking cloud spells.

In addition, Styx dragons also had the ability to summon giant fiendish water creatures such as crocodiles, octopi, sharks, squids, tojanidas, or even water elementals, in an effect similar to the summon monster VIII spell.

Society
Styx dragon lairs were usually dug on the muddy banks of the Styx, or, if brought forcefully into the Prime Material plane, any place with foul water. They could be found alone or in families as they raised their young. Clutches of young Styx dragons were also common.

They could be found on the first layer of all Lower planes of the Great Wheel cosmology, following the course of the Styx, as well as all the Fiendish planes of the World Tree cosmology, following the course of the River of Blood.