Because this is an older mod, it is most commonly found on legacy hosting sites or within specific modpacks:

Every boss has at least one vulnerability. The Ender Lord can be defeated by staying in two-block high spaces. The Lightning Spirit requires explosives or the creative Sharpness V trick. The Elf Hunter must be fought with arrows—bring plenty of them.

To aid in your search, the mod introduces the . Once you defeat one boss, it drops this eye. Similar to an Eye of Ender, you can throw the Sacred Eye into the air, and it will float toward the nearest legendary beast’s lair. This mechanic allows you to hunt down the remaining bosses systematically. 4. Powerful, Unique Loot

Ender Staff, Sacred Eyes.