Amphibious. The swamp snake can breath air and water.

Multiple Heads. The swamp snake has two heads. While it has more than one head, it  has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.

Whenever the swamp snake takes 16 or more damage in a single turn, one of its heads dies. If all its heads die, the swamp snake dies.

At the end of its turn, it grows two heads for each of its heads that died since its last turn, unless it has taken fire or acid damage since its last turn. The swamp snake regains 10 hit points for each head regrown in this way.

Fifth Head. The swamp snake cannot grow more than five heads at a time. The fifth head is always innately more powerful and intelligent than the rest, with an intelligence score of 9, and charisma score of 14. Its innate spellcasting ability is charisma (spell save DC 12, +4 to hit with spell attacks). This swamp snake can innately cast the following spells, with no material components:

At will: Poison Spray, Thorn Whip
2/day each: Faerie Fire, Thunderwave
1/day each: Spike Growth

Frenzied Heads. For each head the swamp snake has beyond one, it gets an extra reaction that can be used only for opportunity attacks.

Wakeful. While the swamp snake sleeps, at least one of its heads is awake.

Grasper. Swamp snakes hone in on a single target, and grabs it while its heads go into a biting frenzy. If the swamp snake fails its first attempt to grapple a creature, and it moved towards that creature this turn, it may hap-hazardously attempt with its other hand, falling prone on success or failure. Whenever the swamp snake has a creature grappled, its speed is reduced to 5 ft.

Multiattack. The swamp snake makes as many bite attacks as it has heads, a crush attack against the creature it has grappled, and cast a single spell if it has grown a fifth head.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 9 (1d10 + 4) piercing damage.

Crush. The swamp snake crushes the creature it has grappled. which must succeed a DC 14 Constitution saving throw or take 10 (2d6+4) bludgeoning damage.