This mod add 17 Hermit Bosses you need to fight. In order to do so, you'll need to explore and find each one of the 17 blocks needed to summon a Hermit Boss.
There block generate on different places, biomes or are droped by special mobs.
You will need to craft a Hermit Summoner then drop the corresponding block on top of it, then, the Hermit Boss will appear and will attack you with his/her own style.
Everyone has a special weakness to be defeated with, of course it makes sense.
Next I will include some information on how to find the blocks you need:
Iskallium ore: find this orde down in the depths, is green and glowing, rarer than diamond.
Stimpack chest: you may find this chest full of healing items on nether fortresses.
Golden log: Look close in the oak trees, there may be a golden piece of log.
Builder's Crate: Have you seen one close to a village blacksmith? There's rarely one there.
Tea plant: This is a rare plant that grows in the jungle.
Blue Slime Block: If you face a blue slime hopping on slime chinks, you may get a chance to get it to drop one of this blocks.
Zombie Flesh Block: In the nether, rarely a lady zombie spawns holding one of these, you can try and get her to drop it.
Crown in stone: Look for desert temples, you may nead to tear them all down and see if there's a crown in the stone.
Snake leaves block: Look for it in the swamplands, can you hear it hiss? Remember to use shears.
@ Block: You may be lucky to find it on an end city.
Castle Diorite: It's high in the mountains.
Cat liter: Special kitty sand you may find on a beach biome.
Cosmic Dust: Falls from the sky, but does not get to reach normal land, maybe in a high mountain.
Crafted Bench: Some villages have fancy crafting tables on their libraries.
Coffee Beans: Are they cocoa beans? No, Coffee beans, look almost the same.
Ocean Glass: On the ocean floor, hold your breath.
Helmet in Stone: Remember those desert wells? Knight helmets are some times in the sandstone.