Avatar of Filth

Undead Biology. The Avatar of Filth does not need to breathe due to its undead nature.

Viscous Form. The dragon can squeeze through openings as small as 5 feet wide. As long as its skull will fit through the opening, the rest of the creature can fit through treating the area as difficult terrain. 

Swamp Strider. The Avatar of Filth's movement is not affected by difficult terrain from the swamp or its' own sludge.

Legendary Resistance. (2/Day) If the Avatar of Filth fails a saving throw, it can choose to succeed instead.

Multiattack. The dragon uses it Aura of Filth and then makes three attacks: one with its bite and two with its claws.

Aura of Filth. The Avatar of Filth spreads its putrid wings and scatters an aura of filth into the air. Any creature within 80 ft of the Avatar of Filth must make a DC 17 Constitution saving throw or be Poisoned for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to Aura of Filth for the next 24 hours.

Bite. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 2d10 + 4 piercing damage plus 1d8 acid damage.

Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 2d6 + 4 slashing damage, plus 2d6 poison damage.

Sludge Jet (Recharge 5–6). The Avatar of Filth exhales acid in a 30­-foot line that is 5 feet wide. Each creature in that line must make a DC 17 Dexterity saving throw, taking 17d6 acid damage on a failed save or half as much damage on a successful one.

Avatar of Filth has 3 Legendary Actions.

Sludge-Bomb (Cost 2 actions). The Avatar of Filth spits a ball of sticky sludge at a point it can see within 30 feet. All creatures in a 15 foot radius must succeed on a DC 17 Strength saving throw or take 4d8 acid damage on a failed save and be knocked prone. The area of sludge is considered difficult terrain until it disperses on the Avatar of Filth's next turn. 

Bone Spear (Cost 1 action). The Avatar of Filth fires a spear made of bones contained within its body. Ranged Weapon Attack:  +7 to hit, range 30 ft, one target. Hit: 2d6 + 4 piercing, and 2d6 acid damage.