Source: The Final Wish, pg(s). 82-83
A zahhak looks like a vast 35-foot-long serpent made of the purest, most malevolent darkness, and moves with a deadly quiet, slithering grace. While for the most part a zahhak looks snake-like, there are some major differences: first, its head is far too ostentatious for a snake, wreathed with many large, curving horns that mark the zahhak as a dragon. Several feet down the length of its body, a zahhak has two huge arms with claws designed to shred rock. From the shoulders sprout the zahhak's most unusual feature: two snakes appear to sprout from its body, each extending down several feet of writhing length to a snake head. While these may look like individual creatures, they are merely appendages of the zahhak. Further down its serpentine body, two large but completely unused wings sprout from the zahhak's back.
Habitat and ecology
Zahhaks tend to dwell in warm hills, preferably amongst ruined forts or other such sinister locations. They are a bigger threat in the distant lands of Kelesh than they are in Avistan or Garund, but can potentially be found anywhere there is desert. Zahhaks are part of a group of dragons known as azi, who combine the worst aspects of dragons and the fiendish div. This mixed heritage gives the zahhak both a venomous flaming breath and the ability to utter curses that strike flying creatures from the sky, causing them to plummet to their deaths.
Zahhaks are the most powerful of all the azi, and are named in honour of one of the dark powers that created them, the draconic deity Dahak. Despite being amongst the physically strongest of the azi, zahhaks are not the smartest, and several legends tell of clever heroes who have used the azi's greed or hateful hunger against them.
One unique aspect that zahhaks have gained from their div heritage is the ability to grant wishes. Much like the most powerful div (who are made from the spirits of corrupted genies), they delight in perverting a person's wishes to destroy their dreams and ruin them.
As the most powerful of the azi, zahhaks are used to ruling over everything they see, and are at heart cruel tyrants of the worst sort. Zahhaks normally claim large territories that often include settlements, which they run like all-powerful tyrants, lording over their slaves and demanding greater and more terrible tribute until they bring complete ruin to their suffering subjects. Zahhaks are also powerful enough to claim power over even more impressive creatures than mere humans: many have their own Usij cult dedicated to them, they often command many different types of div, and some even have other azi who serve them. Even the dead serve the zahhaks, because any humanoid they slay rises within a minute as a ghoul completely loyal to the zahhak who created it.