Caltris, the Black Scourge

Amphibious. The dragon can breathe air and water.

Plaguebringer. Caltris exudes a 15ft emanation of acrid black spores and bacteria. Creatures within this emanation at the beginning of Caltris's turn must succeed a DC20 Constitution saving throw. On a failure, the creature is Poisoned for 1 minute, and takes 16 (4d8) Acid damage. Creatures who fail this save must succeed on an additional save or suffer from Blackrot. 

Blackrot (disease). Blackrot takes 1 day to incubate in the host creature. During this time, creatures feel pain in their joints and respiratory systems, a strange, pulsing headache, and often suffer auditory and visual hallucinations. Identifying the disease and the cure can be done with a DC14 Medicine check. After the disease has incubated, it can only be cured by Greater Restoration or similar magic, or with its identified cure. Creatures within whom the full disease incubates suffer the following effects until it is cured: 

• Restless: The creature cannot benefit from the effects of a Short rest. Long Rests taken by the creature are considered Short Rests. 
• Painful Joints: The creature's movement speed is halved. It takes 1d8 Necrotic damage for every 10ft. moved past half its movement speed. This damage ignores resistances, and immunity is treated as Resistance. 
• Scale Rot: Creatures who have their armor class increased by racial features, or who have increased their armor class by natural means, have their armor class reduced by 1. 

Legendary Resistance (3/Day, or 5/Day in Lair). If the dragon fails a saving throw, it can choose to succeed instead.

Multiple Heads. Caltris begins combat with 3 heads. Whenever he takes 40 damage or more on a single turn, one of his heads dies. Caltris dies if all his heads are dead. At the end of each of his turns when he has at least one living head, Caltris grows two heads for each of his heads that died since his last turn, unless he has taken more than 20 fire damage since his last turn. Caltris regains 30 Hit Points when he grows new heads.

Multiattack. Caltris makes three Rend attacks. It can replace one attack with a use of Spellcasting to cast Melf’s Acid Arrow (level 4 version).

Rend. Melee Attack Roll: +15, reach 15 ft. Hit: 17 (2d8 + 8) Slashing damage plus 9 (2d8) Acid damage.

Plaguebreath (Recharge 5–6). Dexterity Saving Throw: DC 20, each creature in a 90-foot-long, 10-foot-wide Line. Failure: 67 (15d8) Acid damage. The creature must additionally succeed a DC20 Constitution saving throw or contract Blackrot. Success: Half damage.

Spellcasting. The dragon casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 21, +13 to hit with spell attacks):

At Will: Detect Magic, Fear, Bestow Curse (level 4 version), Stinking Cloud
1/Day Each: Create Undead, Speak with Dead, Vitriolic Sphere (level 5 version)

Meld (Recharge 5-6). Caltris melts into the thick, black spore smog. He manifests in an area of his choice within 60ft.

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature's turn , the dragon can expend a use to take one of the following actions. The dragon regains all expended uses at the start of each its turns.

Cloud of Insects. Dexterity Saving Throw: DC 21, one creature the dragon can see within 120 feet. Failure: 20 (4d10) Poison damage, and the target has Disadvantage on saving throws to maintain Concentration until the end of its next turn. Failure or Success: The dragon can’t take this action again until the start of its next turn.

Frightful Presence. The dragon uses Spellcasting to cast Fear. The dragon can’t take this action again until the start of its next turn.

Pounce. The dragon moves up to half its Speed, and it makes one Rend attack.

Spore Eruption (Recharge 6). Constitution Saving Throw: DC 21, All creatures within 15ft. Failure: 24 (6d8) Poison damage.