This is an addon mod for both Stellarity and Boss Checklist that integrates the former's Empress of Light boss into the latter's boss checklist interface as an entry.

"How did you make this work? Stellarity is a data pack bundled as a mod!"

To begin with, the mod adds a fake, unspawnable stand-in entity that acts as the foundation for the boss checklist entry you see. The custom drops displayed in the "drops" category in the entry are likewise, fake, unobtainable items designed to replicate the real deal. The boss's entry is automatically marked as defeated once the server detects an instance of the boss assuming its death animation.

"Why isn't this bundled with your other Boss Checklist addons?"

This mod uses an aforementioned fake entity that is added by this mod itself. Thus, the Empress of Light boss entry will appear regardless of whether or not you actually have the Stellarity mod installed. You wouldn't want a modpack where players would go around curiously searching for this "Empress of Light" only to figure out later down the line that they do not even have the mod supposedly accommodating the boss installed in the first place.

All textures used in representation of the item drops from the boss is directly derived from the Stellarity Resource Pack and aren't of my own making.

Boss Checklist - Stellarity Compatibility