Fey Magic.
Hag Dragons are created by powerful hag covens. The Hags enact a ritual using a Dragon egg of any variety to create a twisted abomination much more dangerous than any true Dragon. Once the ritual is complete, the egg hatches and a Hag Dragon wyrmling is born. It may have some features reminiscent of the Dragon egg’s original species, but they are simply a vestigial remnant of the creature's original form.

Timeless.
Unlike other Dragons, Hag Dragons do not age with the passage of time. They would theoretically remain as wyrmlings forever if it weren’t for the uncanny magical abilities they possess. Hag Dragons can only grow up by absorbing the remaining years of other creatures. By making use of their breath weapon, they not only cause other creatures to age rapidly, but allow themselves to grow incredibly fast. Unlike many other creatures, Dragons only seem to get more powerful as they age, so as the Hag Dragon increases its age it becomes incredibly dangerous in a short amount of time. Many Hags seek to exploit this Trait by keeping Hag Dragons young and easy to control, however, once a Hag Dragon is capable of breaking free from its creators it often does so in violent fashion if it was not treated with respect.

Covens.
While Hag Dragons Covens are incredibly rare, it has been theorised that should a group of them ever come together and agree to cooperate, they could form a coven. In much the same way a group of Hags in a coven become powerful in the extreme, a coven of Hag Dragons would surely be capable of Casting earth shattering rituals. Should such a coven ever arise, it could Spell doom for entire kingdoms if left unchecked.

Covens
A coven of Hag Dragons consists of three Hag Dragons in cooperation. When all members of the coven are within 60 feet of one another, they can cast the following Spells by sharing Mana.
→ 1st level - Fog Cloud, Hideous Laughter
→ 2nd level - Crown of Madness, Darkness Sphere, Locate Object, Wither and Bloom
→ 3rd level - Animate Dead, Counterspell
→ 4th level - Arcane Eye, Puzzling Confusion, Charm Monster
→ 5th level - Animate Objects, Cone of Cold, Illusory Twinning
→ 6th level - Eyebite, Move Earth
→ 7th level - Finger of Death, Forcecage
→ 8th level - Control Weather, Horrid Wilting, Maddening Darkness
→ 9th level - Imprisonment, Mass Polymorph, Time Stop, True Polymorph, Wish