Dracolich
Even as long-lived as they are, all dragons must eventually die. This thought doesn't sit well with many dragons, some of which allow themselves to be transformed by necromantic energy and ancient rituals into powerful undead dracoliches. Only the most narcissistic dragons choose this path, knowing that by doing so, they sever all ties to their kin and the dragon gods.
Dracolich Template
Only an ancient or adult true dragon can be transformed into a dracolich. Younger dragons that attempt to undergo the transformation die, as do other creatures that aren't true dragons but possess the dragon type, such as pseudodragons and wyverns. A shadow dragon can't be transformed into a dracolich, for it has already lost too much of its physical form.
When a dragon becomes a dracolich, it retains its statistics except as described below. The dragon loses any trait, such as Amphibious, that assumes a living physiology. The dracolich might retain or lose any or all of its lair actions or inherit new ones, as the DM sees fit.
Type: The dracolich's type changes from dragon to undead, and it no longer requires air, food, drink, or sleep.
Damage Resistance: The dracolich has resistance to necrotic damage.
Damage Immunities: The dracolich has immunity to poison. It also retains any immunities it had prior to becoming a dracolich.
Condition Immunities: The dracolich can't be charmed, frightened, paralyzed, or poisoned. It also doesn't suffer from exhaustion.
Magic Resistance: The dracolich has advantage on saving throws against spells and other magical effects.
Beyond Death
A dracolich retains its shape and size upon transforming, its skin and scales drawing tight to its bones or sloughing away to leave a skeletal form behind. Its eyes appear as glowing points of light floating in shadowy sockets, hinting at the malevolence of its undead mind.
Though many dragons pursue vain goals of destruction and dominance, dracoliches are more nefarious than the most evil dragons, driven to rule over all. A dracolich is a fiendishly intelligent tyrant that crafts complex webs of foul schemes, attracting servants motivated by greed and a lust for power. Acting from the shadows and actively plotting to keep its existence a secret, a dracolich is a cunning and challenging foe.
Dracolich Phylacteries
Creating a dracolich requires the cooperation of the dragon and a group of mages or cultists that can perform the proper ritual. During the ritual, the dragon consumes a toxic brew that slays it instantly. The attendant spellcasters then ensnare its spirit and transfer it to a special gemstone that functions like a lich's phylactery. As the dragon's flesh rots away, the spirit inside the gem returns to animate the dragon's bones.
If a dracolich's physical form is ever destroyed, its spirit returns to the gem as long as the two are on the same plane. If the gem comes into contact with another dragon's corpse, the dracolich's spirit can take possession of that corpse to become a new dracolich. If the dracolich's spirit gem is taken to another plane, the dracolich's spirit has nowhere to go when its undead body is destroyed and simply passes into the afterlife.