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

"How did you make this work? Annihilation Recreated 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 drop displayed in the "drops" category in the entry is likewise, a fake, unobtainable item designed to replicate the real deal. Aside from the Corrupted Trophy replica, there are three other custom items that act as display icons which upon hovering your cursor over, will detail the loot chances apropos of what might appear within the chest that spawns upon the Annihilation's defeat according to each difficulty. The boss's entry is automatically marked as defeated once a player is detected to have earned the advancement attributed to defeating the Annihilation.

"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 Annihilation boss entry will appear regardless of whether or not you actually have the Annihilation Recreated mod installed. You wouldn't want a modpack where players would go around curiously searching for this "Annihilation" fella 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 - Annihilation Recreated Compatibility