Alicia the great serpent of decay

Amphibious. The serpent can breathe air and water.

Magic Resistance. The serpent has advantage on saving throws against spells and other magical effects.

Corrosive body. A creature that touches the serpent or hits it with a melee attack while within 5 feet of it takes 4 (1d8) acid damage. Any non-magical weapon made of metal or wood that hits the pudding corrodes. After dealing damage, the weapon takes a permanent and cumulative -1 penalty to damage rolls. If its penalty drops to -5, the weapon is destroyed. Non-magical ammunition made of metal or wood that hits the pudding is destroyed after dealing damage.

Multiattack. The serpent makes two attacks: one with its bite and one with it's tail.

Bite. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 15 (2d10 + 4) piercing damage plus 7 (2d6) poison damage.

tail slam. Melee Weapon Attack: +7 to hit, reach 10 ft.,  target in a line. Hit: 11 (2d6 + 4) bludgeoning damage.

decaying Breath (Recharge 3–4). The serpent  exhales poisonous gas in a 30-foot cone. Each creature in that area must make a DC 14 Constitution saving throw, taking 25 (8d6) poison damage on a failed save, or half as much damage on a successful one.

Wail of the swamp  the serpent lets out a harrowing cry, spending a turn concentrating on the most disturbing sound it can conjure. If it's concentration is not broken, any creature within earshot is aged by 1/8th of the target's lifespan.