This is an addon mod for both Ultris: Boss Expansion and Boss Checklist that integrates all of the former's bosses into the latter's boss checklist interface as entries.

"How did you make this work? Ultris is a data pack bundled as a mod!" To begin with, the mod adds fake, unspawnable stand-in entities which act as the foundation for the boss checklist entries you see. The custom drops displayed in the "drops" category in each entry are likewise, fake, unobtainable items designed to replicate the real deal. Each entry is automatically marked as defeated once a player is detected to have earned the advancement attributed to defeating the respective bosses.

"Why isn't this bundled with your other Boss Checklist addons?" This mod uses aforementioned fake entities that are added by this mod itself. Thus, all boss entries added by this mod will appear regardless of whether or not you actually have the Ultris: Boss Expansion mod installed. You wouldn't want a modpack where players would go around curiously searching for a "Sanctum Keeper" 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.

Super thanks to Yori3o for actually heeding my suggestion of adding data pack support to their mod! <3