Yffirion was once a treacherous and ruthless green dragon, but while Yffirion slept he was pulled into the shadowfell. His glorious green scales and crest were burned away by the necrotic energy of the shadowfell when he returned to the material plane he found that he was incorporeal and vulnerable. Through guile Yffirion managed to convince a group of mortals that the only way that they could defeat the spectral plague is to build a cage for it, however Yfirrion had influenced a designer and changes were made that turned the cage into a suit of armor. The mortals thought that Yffirion was defeated when he was forced into the armor, but Yffirion proved that they had only made him stronger. Yffirion earned the name Plague Bringer over the next several years until a group of adventurers managed to seal Yffirion in a vault under the Miskath pit. 

Yffirion chooses a lair that plays to his strengths, somewhere dark

Lair Actions

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

• Grasping shadows erupt in a 20-foot radius centered on a point on the ground that the dragon can see within 120 feet of it. That area becomes difficult terrain, and each creature there must succeed on a DC 15 Strength saving throw or be restrained by the shadows. A creature can be freed if it or another creature takes an action to make a DC 15 Strength check and succeeds. The shadows return to normal when the dragon uses this lair action again or when the dragon dies.
• A wall of tangled brush bristling with thorns springs into existence on a solid surface within 120 feet of the dragon. The wall is up to 60 feet long, 10 feet high, and 5 feet thick, and it blocks line of sight. When the wall appears, each creature in its area must make a DC 15 Dexterity saving throw. A creature that fails the save takes 18 (4d8) piercing damage and is pushed 5 feet out of the wall’s space, appearing on whichever side of the wall it wants. A creature can move through the wall, albeit slowly and painfully. For every 1 foot a creature travels through the wall, it must spend 4 feet of movement. Furthermore, a creature in the wall’s space must make a DC 15 Dexterity saving throw once each round it’s in contact with the wall, taking 18 (4d8) piercing damage on a failed save, or half as much damage on a successful one. Each 10-foot section of wall has AC 5, 15 hit points, vulnerability to fire damage, resistance to bludgeoning and piercing damage, and immunity to psychic damage. The wall sinks back into the ground when the dragon uses this lair action again or when the dragon dies.
• Magical shadow billows around one creature the dragon can see within 120 feet of it. The creature must succeed on a DC 15 Wisdom saving throw or be charmed by the dragon until initiative count 20 on the next round.

Regional Effects

The region containing a legendary green dragon’s lair is warped by the dragon’s magic, which creates one or more of the following effects:

• Thickets form labyrinthine passages within 1 mile of the dragon’s lair. The thickets act as 10-foot-high, 10-foot-thick walls that block line of sight. Creatures can move through the thickets, with every 1 foot a creature moves costing it 4 feet of movement. A creature in the thickets must make a DC 15 Dexterity saving throw once each round it’s in contact with the thickets or take 3 (1d6) piercing damage from thorns.
  Each 10-foot-cube of thickets has AC 5, 30 hit points, resistance to bludgeoning and piercing damage, vulnerability to fire damage, and immunity to psychic and thunder damage.
• Within 1 mile of its lair, the dragon leaves no physical evidence of its passage unless it wishes to. Tracking it there is impossible except by magical means. In addition, it ignores movement impediments and damage from plants in this area that are neither magical nor creatures, including the thickets described above. The plants remove themselves from the dragon’s path.
• Rodents and birds within 1 mile of the dragon’s lair serve as the dragon’s eyes and ears. Deer and other large game are strangely absent, hinting at the presence of an unnaturally hungry predator.

If the dragon dies, the rodents and birds lose their supernatural link to it. The thickets remain, but within 1d10 days, they become mundane plants and normal difficult terrain, losing their thorns.