Here's a custom boss I've worked on for someone's map.
It's a phantom that ressembles a giant manta ray.

This baby took a good three months to build,
with over 150+ functions in total.

This is my first official project on Java Edition,
and the results don't look too bad in my opinion.

Important notes

This datapack requires a world to function properly.
Because the upload limit is 10Mo, you will need to follow these steps.

First, you will need to find the "MC-Datapacks" repository on GitHub. After that, you will have to download the zip file which includes the '_worlds/' folder.

Once the codebase is downloaded on your PC, unzip it.

Then, you would need to unzip and insert the 'boss_water_temple.zip' datapack into the 'Water Boss/datapacks/' subfolder.

Finally, all you will need to do is to add the "Water Boss" folder in your Minecraft saves.
This can be accessed by entering %AppData% in your search bar, then '.minecraft/saves'.

If you have any questions about this datapack, leave a comment.

Enjoy !