Immunity. Acidag cannot take poison or acid damage. He also has a swim speed equal to his walking speed.

Spellcasting. Acidag is a 4-level spellcaster. Its spellcasting ability is Constitution (spell save DC 15, +5 to hit with spell attacks). Acidag has the following spells prepared:
Cantrips (at will): Acid Splash, Poison Dart 
1st level (2 slots): Venomous Strike
2nd level (2 slots): Breath of the Green Dragon
3rd level (2 slots): Toxic Rain

Adagcid. Melee Weapon Attack: +5 to hit, reach 5 ft., 1 target. Hit: 18 (3d10 + 3) poison or acid type damage. 

Adagcid Sword Form. Melee Weapon Attack: +5 to hit, reach 7 ft., 1 target. Hit: 18 (5d10 + 3) poison or acid type damage. 

Acid Blood. Whenever a creature makes an attack against Acidag they must succeed on a constitution saving throw or take 1d4 acid damage.

Acidag can take 3 Legendary actions per day.

Stone Infection (Costs 1 Action).  Acidag can infect one target he can touch. Until the creature is completely infected a jade circle spreads outward from the point of contact. The jade, when covering anything is immovable. It takes 10 minutes for each size class of the target to become completely infected. The target then becomes paralyzed and their equipment becomes jade as well. 

Poison Pump (Costs 2 Actions). Acidag chooses one creature he has made a melee attack against that creature must succeed on a DC 15 Constitution saving throw or have the following effects take place. 1st fail: the target takes 1d4 poison damage, 2nd fail: the target's nonmagical equipment melts and the target takes 1d4 acid damage, 3rd fail: the target is poisoned and takes 1d4 poison damage, 4th fail: the target's flesh on their legs begins to burn off and they take 1d4 acid damage, 5th fail: the target's flesh on their torso begins to burn off and they take 1d4 poison damage, 6th fail: the target's flesh on their arms begins to burn off and they take 1d4 acid damage, 7th fail: the target's flesh is completely burned off they take 2d6 acid damage and become a skeleton. The target now has 3 choices, they can be cured by remove curse or revival and resurrection, they can remain a skeleton, or they can spend 3 long rests to become a bonewalker.