Bubble Beast

Amphibious. The Bubble Beast can breathe in both air and water

Unpoppable. The Bubble Beast hides inside a large bubble made of magically reinforced water. It has total cover and loses 10ft. of walk speed while the bubble holds. The bubble itself has 5 hit points and is immune to all damage types that aren't frost, piercing, or lighting. Lightning damage hurts the beast without touching the bubble. Only the Bubble Beast can freely move itself and others through the bubble's surface. Whenever the bubble is attacked by a damage type it is immune to, it is thrown back a distance equal to 15ft. for every 10 damage that would've been dealt.

Spider Grab. Melee Weapon Attack: +2 to hit, reach 5 ft., 1 target. Hit: 7 (2d6) piercing damage. If the target is a creature, it is brought inside the bubble and grappled (escape DC 12). Until this grapple ends, the target is restrained, and the Bubble Beast can't use its Spider Grab on another target. If it escapes, the target is immediately released from the bubble.

Bubble Sculpt. The Bubble Beast starts concentrating on making a new bubble in which to hide. If this concentration holds until the start of the Bubble Beast's next turn, a new bubble is formed. This action can only be done while in or next to a body of fresh water.

Flee. The Bubble Beast may make the Dash action as a bonus action. If it does this, its bubble breaks if it hasn't already done so.