All adventurers worth knowing know of the singular superiority over others that a dragon takes as granted as the air we breathe. But a knowledge given to "lesser" races is that one learns best when they do not act as though they are the greatest in the room. Rare is the dragon that will learn from others. Rarer still is the one that associates with the Yuan-Ti. Where these qualities and a determination for power combine is the use of Yuan-Ti conversion rituals on a dragon.

When these rituals are performed with thousands more snakes and hundreds more human sacrifices, they create Yuan-Ti Wyrms. Whatever the dragon's previous identity, it is washed away as their scales turns emerald green, their belly moves so close to the floor that they slither instead, their legs absorb into their skin, their arms become smaller but more prehensile, their heads become smaller and flatter at the top but filled with venom, their skin becomes wet to the touch, and their minds become more power-hungry than the worst Anathema. The Yuan-Ti gods, however, make certain these powerful beasts do not mantle and kill them by giving them an inability to resist Charming.

Still Dragons. Dragons are still dragons, even if they have become more ophidian than draconic. Wyrms tend to collect magic objects that can control the minds of others, out of frustration at their enslavement and out of a desire to impress it upon others. Any item that has an ophidian aesthetic also fills their hoards, but what particularly catches their eye are any books of knowledge on how to mantle the serpent gods. Their following of the serpent deities is almost exclusively for the benefits of the power it brings, and when a Yuan-Ti Wyrm uncovers the ability to mantle the gods themselves, the natural order is upset beyond any possible return to the status quo.

It is not enough for the Wyrm to embrace its new snake nature; its environment must reflect these changes, as well. Within the wyrm's lair, hundreds, maybe several thousands of snakes slither across its floor, paralyzing any would-be trespassers with an ocean of bites filled with neurotoxins. The air corrodes the senses, and all that one can smell are the corpses of victims soon to be engorged. Every room the wyrm does not esteem eventually corrodes into alien geometry as the absentminded droolings of the wyrm's panting mouth creating acidic saliva hit the floor. Blood stains are the decor of choice, usually streaked in places that a snake's body cannot reach. These jungle abodes may be surrounded by life, but as soon as a piece of living organic matter dares to grow within the radius, only the hardiest of creatures can survive.

Lair Actions

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

• A clutch of snake eggs hatches in response to the violence in its area, releasing a swarm of poisonous snakes into the battle. These snakes automatically have an Initiative of 20 and have an Ophidiophobia aura with a range of 30 feet. They also have an HP of 60 and will automatically take damage for any friendly ally within 5 feet that is not another swarm of poisonous snakes. Excess damage hits the original target.

• A cloud of swarming insects fills a 20-foot-radius sphere centered on a point the wyrm chooses within 120 feet of it. The cloud spreads around corners and remains until the wyrm dismisses it as an action, uses this lair action again, or dies. The cloud is lightly obscured. Any creature in the cloud when it appears must make on a DC 15 Constitution saving throw, taking 10 (3d6) piercing damage on a failed save, or half as much damage on a successful one. A creature that ends its turn in the cloud takes 10 (3d6) piercing damage.
• Magical darkness spreads from a point the wyrm  chooses within 60 feet of it, filling a 15-foot-radius sphere until the wyrm dismisses it as an action, uses this lair action again, or dies. The darkness spreads around corners. A creature with darkvision can’t see through this darkness, and nonmagical light can’t illuminate it. If any of the effect’s area overlaps with an area of light created by a spell of 2nd level or lower, the spell that created the light is dispelled.

Regional Effects

The region containing a legendary Yuan-Ti Wyrm's lair is warped by the wyrm's magic, which creates one or more of the following effects.

• Snakes reproduce like weeds, and become unnaturally cruel. There is disadvantage on all Animal Handling checks to appease snakes and swarms of poisonous snakes are considered to have twice their normal HP. They also deal twice their rolled poison damage on every hit and targets must beat a DC 10 Constitution check or be paralyzed by their poison for one round.
• Acid rain falls within 5 miles of the lair when rain naturally falls, killing all plants that are sensitive to pH changes. Every 2 uninterrupted hours in the rain, a creature takes 1d4 Acid damage. Long rests are not possible unless shelter is taken.
•  Dead bodies within 5 miles of the lair do not rot and can be restored even by Revivify so long as they remain within the radius. They do, however, emit cloyingly sweet and simultaneously meaty smells that attract snake swarms to the bodies 2d6 minutes later. If the bodies are consumed by the snakes, the souls of the bodies are immediately taken up to the snake gods and consumed painfully, making resurrection impossible even by a Wish spell.

If the wyrm dies, vegetation remains as it has grown, but other effects fade over 1d10 days.