Lightning Suceptability. If the jellyfish takes lightning damage its speed becomes 0 until the end of its next turn.
Amorphous. The jellyfish can move through a space as narrow as 1 inch wide without squeezing.
Poisonous Body. Whenever the jellyfish is hit or missed by a melee attack, the attacker must make a DC 15 dexterity saving throw or take 10 (2d10) poison damage.
Multiattack. The giant jellyfish makes 2 tentacle attacks.
Tentacle. Melee Weapon Attack: +6 to hit, reach 10 ft., 1 target. Hit: 6 (1d6 + 3) bludgeoning damage and 20 (4d10) poison damage. The target is grappled and must then make a DC 15 constitution save or be paralyzed for 1 minute. It may repeat the saving throw at the end of each of its turns.
Liquid Body. When the jellyfish would take damage from a weapon attack, it can use its reaction to contort its form and avoid damage altogether, reducing damage from the triggering attack to 0.