The abyssal dragon is a massive dragon-like creature with dark purple and black scales. Its eyes glow like red hot coals, and its fangs drip with venom. Its voice can shake the earth, and all abyssal creatures tremble in fear when the abyssal dragon is near. It rules as King over its layer of the Abyss, which appears to mirror the Material Plane, but everything is more distorted and dark. 

Thousands of years ago, when the First World still existed, the abyssal dragon was nothing but a normal dragon. It was significantly stronger than most other dragons, but nothing close to the Dragon Queen Tiamat. It hated the Dragon Queen, but could do nothing against her. Until one day, the First World was invaded. Many gods and their races came for the First World, claiming it as their own. Tiamat fought fiercely, but was outnumbered. She was defeated and banished to Avernus, the first layer of the Nine Hells. The abyssal dragon had little time to rejoice before it was targeted next. It fought as hard as it could, but then a god came out of nowhere and banished the abyssal dragon to the Abyss. It plummeted down through the infinite layers, colliding with many demons on the way. The demons were absorbed into the abyssal dragon as it fell, and when it finally landed it had greatly changed.

The abyssal dragon was sure that with its new power it could finally take down the Dragon Queen, but though it tried as hard as it could, it wasn't able to escape the Abyss. It now rules over the layer it landed on, and lives in a massive cave that was created from the impact of the abyssal dragon hitting the ground.

The abyssal dragon lives in a massive cave, with many huge caverns and ravines. Lava flows through many parts of the cave, providing it with eerie red light. The shadows seem to be alive as they dance in the flickering light of the lava. Earthquakes are common around the lair

Lair Actions
On initiative count 20 (losing initiative ties), the abyssal dragon takes a lair action to cause one of the following effects; the abyssal dragon can’t use the same effect two rounds in a row:

• Magma erupts from a point on the ground the abyssal dragon can see within 120 feet of it, creating a 20-foot-high, 5-foot-radius geyser. Each creature in the geyser’s area must make a DC 15 Dexterity saving throw, taking 21 (6d6) fire damage on a failed save, or half as much damage on a successful one.
• A tremor shakes the lair in a 60-foot radius around the abyssal dragon. Each creature other than the abyssal dragon on the ground in that area must succeed on a DC 15 Dexterity saving throw or be knocked prone.
• Volcanic gases form a cloud in a 20-foot-radius sphere centered on a point the abyssal dragon can see within 120 feet of it. The sphere spreads around corners, and its area is lightly obscured. It lasts until initiative count 20 on the next round. Each creature that starts its turn in the cloud must succeed on a DC 13 Constitution saving throw or be poisoned until the end of its turn. While poisoned in this way, a creature is incapacitated.

Regional Effects
The region containing a legendary abyssal dragon’s lair is warped by its power, which creates one or more of the following effects:

• Earthquakes are very common within 6 miles of the lair.
• Cracks form in the ground, allowing lava to slowly pour out within 1 mile of the lair.

If the abyssal dragon dies, these effects fade over the course of 1d4 years.