Baron of Pestilence (Demolith)

Demolith Subtype. The baron has the "(demolith)" tag, which has no rules of its own, but is a tag that affects him in the same way that it would affect a demon or devil.

Semi-Immortal Nature. The baron can't die of old age unless affected by magic of a spell of 6th level or higher and doesn't require food, drink, air, or sleep.

Lesser Legendary Resistance (3/Day). If the baron fails a saving throw, he can choose to retry with advantage.

Unconstitutional Presence. Creatures within 120 feet of the baron that can hear him have disadvantage on saving throws against being poisoned, paralyzed, or stunned.

Diseased Death. If the baron reduces a creature to 0 hit points, it has disadvantage on all death saving throws and begins to have hideous pustles grow all over its body. This causes any creature that gets within 10 feet of the dying creature must succeed on a DC 15 Constitution saving throw, or be poisoned for 1 minute.

Multiattack. The baron makes one melee attack and two with his Sludge Pool. He can replace both of his Sludge Pools with a Leaping Slam.

Claw. Melee Weapon Attack: +10 to hit, reach 10 ft. Hit: 15 (3d6 + 5) Slashing damage plus 10 (3d6) Poison damage.

Bite. Melee Weapon Attack: +10 to hit, reach 5 ft. Hit: 12 (2d6 + 5) Piercing damage and the target must succeed on a DC 18 Constitution saving throw or contract a random disease of the DM's choice that lasts for 1 hour. The target can repeat the saving throw at the start of each of its turns, doing so with disadvantage, becoming undiseased on a successful save.

Horned Slam. Melee Weapon Attack: +10 to hit, reach 5 ft. Hit: 11 (1d12 + 5) Piercing damage plus 5 (2d4) Bludgeoning damage. 

Sludge Pool. The baron slams his arm down and creates a 5-foot wide pool of spluttering bile and filth at a point he can see within 30 feet of him. The pool lasts for 1 minute. Any target that starts or ends its turn while within the pool must make a DC 16 Constitution saving throw, taking 7 (2d6) Poison damage and being poisoned for 3 minutes on a failed save, or half as much damage and not be poisoned on a success.

Leaping Slam. The baron leaps high into the air before slamming down at a point he can see within 50 feet of himself. Each target within 20 feet of the baron when they land must make a DC 16 Dexterity saving throw, taking 18 (4d8) bludgeoning damage plus 13 (2d12) poison damage and be knocked prone on a failed save, or half as much on a success.

Blight of Hell (Recharge 6). The baron releases a burst of viscious filth and bile in a 40-foot radius around himself by causing numerous pustles on his body to ferociously explode open. Each target within the area of the attack must make a DC 18 Dexterity or Constitution saving throw (baron's choice), taking 45 (7d12) poison damage and be Poisoned for 3 minutes on a failed save, or half as much damage and only be poisoned until the end of the next round on a success.

Legendary Action Uses: 3. 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 of its turns.

Rend. The baron makes a melee attack.

Blight (Costs 2 Actions). The baron uses Sludge Pool.