While hanging around outside the Construct Inc building, I heard the sounds of zombie regular Zombie groaning. I looked in the direction of the groans and see a 20x20 structure appeared near the curse detector's spawning location outside of the spawn building. It had a strange movement pattern to it, the middle part stretched and sank, as if it was made of water and someone dropped something into the middle of it. The structure also emits a creepy ambient noise and heartbeats.

A zombie appeared underneath the structure. As the Zombie limped towards me, the structure followed above it. I killed the Zombie to see if it would effect the structure in any way. The cubes that the structure was made of started falling to the ground at random and in to particular order after the Zombie was killed.

Here's screenshot of the structure https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2864786413

I didn't have any other addons installed besides the map at the time that this event
occured.

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Updated: I have had my second encounter with the structure. This time I decided to observe it instead of killing the Zombie. The structure floats above the Zombie at all times and seems to be bound to it mid air, wherever the Zombie goes, the structure will follow above it at the same height and directly in the middle.

I let the Zombie follow me through a doorway to see if the structure would collide with the wall, the result being the structure simply no-clipping through the wall and presumably anything else that gets in it's way. The structure cannot be interacted with in any way as far as I know and doesn't have any collision. The structure and the Zombie will disappear if left alone for a certain amount of time.

As Noftasmos mentioned, the cubes that make up the structure will emit a coughing sound whenever they despawn. (Same applies to the structure disappearing after some time has passed without interaction).

The Zombie that the structure is bound to doesn't seem to be any different to a regular Zombie. It's walk speed is the same, it's attack damage is the same, and it's health is the same.