Shadow Dragons are a species of Dragon in the many worlds of Dungeons & Dragons with a strange elemental affinity for darkness, the illusionist arts, and necrotic energy. They are usually a form of Planar Dragon, with ties to the Plane of Shadow or its post-Great Wheel equivalent, the Shadowfell.
They go back incredibly far, having first appeared in Advanced Dungeons & Dragons 1st edition's Monster Manual II. They then made the jump to 2nd edition in the Monstrous Compendium Greyhawk Appendix, with lore from that being reprinted in 1993's Monstrous Manual. In 3rd edition, like several other Greyhawk monsters, they migrated to the Forgotten Realms, first appearing in "Monsters of Faerun" before being reprinted, sans Realms-specific notes, in the 3e Draconomicon.
Finally, in 5th edition, it hit the big time and graduated to a debut appearance, showing up in the Monster Manual of this edition. Here, however, they lost their status as a unique dragon species and were instead reinterpreted as a template, based on a "true dragon" (Chromatic, Metallic, etc) spending too long in the Shadowfell and being corrupted into a partially shadow-based version of their former selves. This transformations makes the Dragon stealthier, highly resistant to damage while in darkness, and sensitive to sunlight, while granting it a breath weapons that animates those it slays into undead shadows.
Shadow dragons are sly and devious creatures with ties to the Plane of Shadow. Shadow dragons have translucent scales and dark bodies, giving them an indistinct appearance; from a distance, they seem to be nothing but a foreboding mass of shadows.
Shadow dragons prefer to attack from hiding, employing their shadow blend ability. They use illusion spells to confuse and misdirect their foes.
In the Realms, shadow dragons are most often encountered in the deepest reaches of the lower Underdark —at least ten miles below the surface— where ties to the Plane of Shadow are strongest. On the surface, they have been found from the Frost Hills to the Thunder Peaks. A number of shadow dragons have joined forces with the Cult of the Dragon, and at least two shadow dracoliches are known to be associated with the Cult.