Category 5 Hydroid Krasis

Amorphic. The Krasis is able to change shape in order to fit through spaces 1 inch thick at smallest. The Krasis always changes shape to fit into its surroundings. There are 3 specific amorphic forms available to the Krasis that affect its AC and max HP. All HP loss is carried over from the previous amorphic shape, even if this would result in the death of a Krasis. 

TINY (mouse-sized) - Max HP: 30, AC: 17. When tiny, the Krasis deals quarter damage.

MEDIUM (humanoid) - Max HP: 100, AC: 16. When medium, the Krasis deals half damage.

GARGANTUAN (base size) - Max HP: 300, AC: 14.

Bioluminescent Magic Disruptors. The Krasis gives off an aura of 40ft radius. Within this aura, magic is disrupted, and the caster must roll a d6 whenever they attempt to cast a spell. On an odd roll the spell fails (a spell slot is still used), on an even roll the spell acts normally.

Absorbent Slime. The Krasis is covered in a slime that is able to absorb ranged magic projectiles. When a creature makes a ranged spell attack against the Krasis, or forces the Krasis to make a saving throw as a result of magic, roll a d6. On an odd roll the projectile hits the slime and dissipates, causing no damage. On an even roll, the slime absorbs the magic and imbues the Krasis with energy, allowing it to add 3d6 force damage to all attacks next turn.

Regenerative. At the start of each of its turns, the Krasis regains 10% of the max HP of its current amorphic form. Additionally, once a Krasis has been reduced to 0 HP, if any body parts are left intact the Krasis can regenerate fully from the body part over 24 hours.

Siege Monster. The Krasis deals triple damage to constructs.

Multiattack. The Krasis makes 4 bite attacks.

Bite. Melee Weapon Attack: +9 to hit, reach 15 ft., 1 target. Hit: 15 (3d6 + 3) piercing damage.