If you're familiar with Terraria, Core Keeper has a very similar boss structure to the game. Many bosses can be found in the world and some need to be summoned. They all drop specific loot. Currently there are 17 bosses in the game and two seasonal variants. Here's all of the bosses in the game.
Glurch is a huge orange slime that can be found in the Undergrounds fairly close to where you start the game. This is the first boss you should fight, but still make sure to come prepared armor and weapons, Glurch may be a slime, but he ain't a slouch.
Ghorm is a boss that can be found in the Clay Caves and Forgotten Ruins. Be care with this one, as Ghorm is a bit faster than the first boss. Make sure you're properly equipped with much more advanced armor and weapons than you were for Glurch as well. Equips are also a great addition to your setup.
The King Slime has somehow gotten into Core Keeper from Terraria and he's here to cause problems in your life. Similar to Terraria, you'll have to summon him with a Crown Summoning Idol on any slime that is summoning rune. He's fairly easy and will just jump at you and spawn some royal slimes. Just beat him to death, it's really that easy.
Found in the Larva Hive, the Hive Mother is a gross monster with tentacles that can be a bit more difficult than the other bosses. It's recommended to use Larva Walls to cage in the spawning points on the ground, so you'll only need to avoid the acid during this fight.
The first more humanoid looking boss in the game (I know, you're sick of fighting bugs and blobs). You can summon him with a Crystal Skull and then place it on the Throne in the Throne Room in the Forgotten Ruins. This boss is optional, but he's not the easiest boss. Make sure you are ready to move swiftly or block his attacks, since he'll shoot projectiles at you.
It's a bird, it's a plane, no, it's Azeos the Sky Titan. It's recommended to use poison on him since he'll be healing during the fight. He also has an attack that can instantly kill you. When the ring of beams is summoned, make suer to stay at the center to make sure you're not killed.
Ivy can be found in Azeos' Wilderness in an area completely covered in slime. Make sure you can act swiftly, as Ivy will fling itself at you quickly with a splash circle of damage around it. It will also drop poison from the sky onto you, so make sure you're set up to be dodging a lot this whole fight.
Found in the Sunken Sea, Omoroth is the first aquatic boss of the game. It will spawn whirlwhinds near it, so it's best to come equipped with ranged weapons for this fight so you can attack at a distance and stay out of them. Watch out for his tentacles as well, which will deal some major damage.
Morpha can be a tough boss because she'll jump at you, then fire bubbles all while the arena is slimy and slippery for your player. The best way around it is to grind Slippery Slimes and get the Slime Armor, which will protect you from slippery movement and let you get around the boss area more easily.
Re-Akar is a trickier boss due to it's agressive attack patterns. To give yourself enough time to dodge or block his attacks, it's recommended to go with a range build for this fight.
Found in the Molten Quarry, Igneous is a giant mass of molten rock and lava. He'll throw fireballs at you as well as spawn blobs of magma that will heal him once they reach him. It's important to kill these before they reach him. Once he is down to about half heath, he will move much faster and throw more projectiles at you.
You can find this worm in the Sunken Seas in a circle, he won't do anything unless you summon it with a Bait Pillar. He will destroy all blocks in his way and replace the ground with water, so keep him away from anything precious. He'll attack until you destroy one of his weak spots, then split into a smaller worm for you to kill.
You can find the first Titan of the endgame in Azeos' Wilderness. You can summon by placing a Wind Organ of Wilderness by him. It will slam itself down at you, fire a beam at you, summon thorns beneath you, and make blocks fall from the sky and create a shockwave in doing so.
Crydra can be summoned using the Wind Organ of Ice next to it. It will do similar attacks sa Druidra, but will also create crystals that shoot ice at you. Just like the other titans, he has a shield to be depleated, and then will be able to be damaged.
Pyrdra can be spawned by placing a Wind Organ of Fire next to it. His abilities are the same as the other Titans, but can spit fireballs into the sky that will fall in a line on the ground. They will also leave magma slime on the ground for you to avoid.
The Final Boss of Core Keeper, the Core Commander. He can be summoned by interacting with and shattering the crystal meteorite once getting all 6 titan souls. To defeat him, you must destroy all of the Core Spheres in the arena, which will leave him vulnerable for attacks. During the time he is invulnerable, he will shoot lighting bolts at you in many different ways.
Finally, we have Urschleim, found in the Passage. Found very far away from the Core, this boss is one of the biggest bosses in the game. You'll want to kill all of it's glowing bulbs that will reveal the worm part pictured below. Use ranged weapons for this to shoot at his bulbs from a distance. He moves in the same way the whole fight, so he's easy to learn and complete.
Those are all of the bosses in Core Keeper. Make sure to grab a Core Keeper server for you and your friends from Indifferent Broccoli so you can beat them all with your friends.