Undine Wyrms are a highly regulated species controlled by the water clan due to the fact that they can be misused to disable any water based species and turn them on their owners. Before this control, undines were quite rare in the wild and hard to tame, but few mages were able to control them. One of these mages realised that their control of water was not just limited to control of lakes and streams, and proceeded to sell these eggs to enemies of the water clan with the full intent of controlling their gem. Thankfully, other clans caught wind of this and realised that the fall of one will lead to the fall of them, and turned the offender in and set out to collect all of the species for captivity.
This hatchling closely resembles a common snake, though it seems to be able to control and move water on its own. Your other water-loving hatchlings enjoy watching it try to control them. It's grown past the size of the usual snake now and even is able to control water to make shapes. Your water-based hatchlings are sometimes put under a trance and can be controlled by it for short periods.
Your hatchling has grown now and its length easily allows for it to span across your room. It has full control over water now and you carefully watch it, as it often will take the chance to control a water based dragon with its powers instead of using them just for hunting and defense. It can hold a very powerful trance over any of these species and you can see now why it was incredibly regulated to only people the clan could trust.