This is an addon mod for both Harrowing Hells, Boss Checklist and Field Guide that integrates the former's Cinderscourge boss into the latter two's boss checklist interface as an entry.

"How did you make this work? Harrowing Hells 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 entry you see. The custom drop (Nether Dwarf Star) displayed as a drop in the entry is likewise, a fake, unobtainable item designed to replicate the real deal. The boss's entry is automatically checkmarked once the server detects that an instance of the boss has died.

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

This mod uses an aforementioned fake entity that is added by the mod itself. Thus, the boss entry added by the mod will appear regardless of whether or not you actually have Harrowing Hells installed. Especially in the case of Boss Checklist, you wouldn't want a modpack where players would go around curiously searching for this "Cinderscourge" 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.

Boss Checklist & Field Guide - Harrowing Hells Compatibility