Death Burst. The Bubble explodes when it drops to 0 hit points. Each creature within 20 feet of it must take 16 (4d6) acid damage. If the bubble is killed buy any damage it has a vulnerability to (exept for peircing) or a  wish spell, it does not expolde.

Transparent. Even when the bubble is in plain sight, it takes a successful DC 15 Wisdom (Perception) check to spot a bubble that has neither moved nor attacked. A creature that tries to enter the bubbles's space while unaware of the bubble is surprised by the bubble.

Engulf. The bubble moves up to its speed. While doing so, it can enter Large or smaller creatures' spaces. Whenever the bubble enters a creature's space, the creature must make a DC 15 Dexterity saving throw.

On a successful save, the creature can choose to be pushed 5 feet back or to the side of the bubble. A creature that chooses not to be pushed suffers the consequences of a failed saving throw.

On a failed save, the bubble enters the creature's space, and the creature takes 16 (4d6) acid damage and is engulfed. The engulfed creature can breathe for two hours, and takes 2 (1d4) acid damage at the start of each of the bubble's turns. When the bubble moves, the engulfed creature moves with it.

An engulfed creature can try to escape by taking an action to make a DC 8 Strength check. On a success, the creature escapes and the bubble pops, entering a space of its choice within 5 feet of where the bubble was.

Burst.  When the bubble takes peircing damage, it instantly drops to 0 hit pionts, causing the death burst trait.