This mod adds multiple bosses. If you kill them, you can get an item! They'll never be easy.
Bosses are summoned as items. You can check the recipe on JEI
There are also summoning items that cannot be obtained with recipes.
Bosses:
-Flaming Berserker
-Ancient Wizard
This boss's summoning item is obtained with a 50% chance of breaking an iron sword
-Merciless Assasin
-Froverlord
-Vulcan
-Ender Ordnance
-Mono Eyed Skeleton
-Crimson Vampire
-Volcanium
-Sand Giant
-Frost Magma
-Aesegull
-Hand Head
-Nameless One
-Prismarine Watcher
-Rib Golem