Ancient Tethered Dragon

The Tethered. Many have speculated the nature of the Tethered as a collective hive mind of various creatures, but the reality is that the Tethered is one singular entity spreading itself slowly and steadily to influence other creatures. Its abilities are limited to whatever it can best mold each vessel into, but each separate creature is all part of one being. It claims to be ageless, timeless, having no beginning and no end, represented by its lack of core being and the physical connection each of the vessels under the Tethered share with one another. This being delights in hoarding information, and has made quite the collection over the several millennia it has spent out in space. The Tethered might be the universe's singular largest library of knowledge in existence, but access to it means submitting oneself to becoming a part of its ever-growing empire.
Perhaps the term that describes the Tethered best is a viral neural network. Those connected to the network experience a free flow of information streamed mentally between all of those connected, information the Tethered has witnessed over its time observing the universe alongside the thoughts and feelings of every being that is a part of the network. Independence does not exist here, arguments do not break out - every vessel thinks and feels as one. Their connection is entirely physical, however, and while it is unclear how long their physical tethers can stretch, it is assumed that there is some barrier that keeps the Tethered from stretching between celestial bodies. These tethers appear to be impervious to damage once the Tethered has established a secure and permanent connection with the vessel, and have been witnessed linking and unlinking from other tethers as they overlap and move.

Tethered Dragon. When a dragon submits to the Tethered, it becomes a horrible weapon for the Tethered to use on any strong-willed outliers who refuse to become one with it. Very few have had the willpower to stand the barrage of attacks that tethered dragons force upon their adversaries. While they have less ability to manipulate the minds of other creatures in the way that other powerful Tethered vessels can, they make up for it with sheer force and debilitating screeches that manifest as thousands of souls absorbed by the Tethered screaming as one. Oftentimes, tethered dragons are used as siege weapons against more powerful strongholds and cities to turn the massive populations of these locations against them.

Curse of the Tethered. The Tethered are always looking to expand their hive mind to anyone that connects to their network.  A creature that succumbs to this curse begins to undergo the horrifying transformation that turns them into a new vessel for the Tethered.
Pronouncement. The Tethered will always attempt to hide the presence of the curse, but tethered creatures will start to realize their thoughts being watched and new information fed into their minds subversively. This curse can only take effect on creatures who are tethered, a dangerous and risky maneuver on its own.
Burden. The character's mind becomes completely accessible to the Tethered, making them aware of the character's thoughts, mood, memories, class features, current hit points, and any levels of exhaustion they may have. Any deception checks the character attempts to make against the Tethered automatically fail. After their next long rest, the character's body starts to become semi-gelatinous and a sticky red mucus exudes from their pores, giving them advantage on checks and saves made against being grappled or restrained and disadvantage on attack rolls and strength based checks and saves. If the curse is not broken by the end of the character's next long rest, they are blinded (outside of the radius of their 60 ft. blindsight), deafened, and lose the ability to speak verbally as their face takes on the void-like quality of the Tethered, absent of all facial features. At this stage, the character's skin becomes a pale white and their limbs lengthen and distend awkwardly, and they also gain resistance to psychic damage as long as they remain cursed. If the curse is still not broken by the end of their next long rest, the character is permanently transformed into a Tethered under the DM's control. Nothing short of a Wish spell can bring them back after this.
Resolution. After being cursed, the tether on the character can still be broken as normal, but it will regrow and attach to the network if left alone for 1 minute. To prevent the reconnection, there must not be a gap that is 1 inch or larger between the character and the Tethered network. Then, and only then, will a remove curse spell sever the tie between the character and the Tethered, breaking the curse.