The Harbinger is a mechanical boss within the Cataclysm mod. This is the only boss that spawns in the Overworld, specifically within the Ancient Factory structure.
Finding the Ancient Factory
The Harbinger spawns within the Ancient Factory. This structure is located in the Overworld and is a dungeon that is found deep underground. So get your best Pickaxe, as you will need to do some mining.
The easiest way to locate the Ancient Factory (and the Harbinger) is to create an Eye of Mech, which requires 4 Redstone Blocks, 4 Iron Ingots, and an Eye of Ender. These are very similar to a standard Eye of Ender.
When holding the Eye of Mech, you can use the right mouse button to release it. It will then float in the direction of the nearest Ancient Factory, so keep doing this multiple times. When the Eye of Mech floats above you, that’s when you should start to dig downwards.
Alternatively, some commands can be entered, provided you have cheats enabled. You can use the commands /locate cataclysm:ancient_factory or /locate structure cataclysm:ancient_factory on Minecraft 1.19+.
Within the Ancient Factory, you can find some blocks that might can be helpful before going into the fight, such as Anvils, Blast Furnaces, and Grindstones. Then, you will see the large arena containing the Harbinger.
Fighting the Harbinger
It’s safe to enter the area that the Harbinger resides in, as it will be shut down. To awaken the Harbinger, you will need to hold a Nether Star and use it on the Harbinger; then, it will wake up, and the fight will begin.
Firstly, the Harbinger has 390 health and 12 armor. And when it fights, it will switch between missile and laser modes. These are easy to counter, provided you have a Shield, and some Fire Resistance, as these attacks will set you on fire.
These attacks will also hit you with the ‘Wither’ effect, so bring plenty of food. It may also be worth bringing Buckets of Milk to remove any negative debuffs if you think it could be helpful. Every so often, the Harbinger will use a new ability, the Death Laser.
After powering up for a few seconds, the Harbinger will release a powerful laser onto enemies, dealing lots of damage. When this attack happens, the best thing you can do is simply run away and keep healing. Sometimes, the Harbinger will do a charge attack, which can destroy blocks within the arena.
Though it’s also possible to use the Death Laser to your advantage, EMP blocks are placed around the room, which can be charged up if the Harbinger hits them with the Death Laser. When these are activated with Redstone, such as the Lever placed alongside them, they will release an EMP pulse.
If the EMP pulse hits the Harbinger, it will be disabled for a few seconds, allowing you to either deal damage or heal yourself. Other than that, the fight shouldn’t be too difficult and only take a minute or two.
Make sure to bring Enchanted Netherite equipment or better. A good weapon choice would be an Axe, or the Infernal Forge, which can be obtained from the Netherite Monstrosity. Because the Harbinger tends to stay in the air, a Bow and Arrow would also be a good idea.
Two items can be obtained from the Harbinger. The first is the Wither Assault Shoulder Weapon. When right-clicking with this weapon equipped, you will fire missiles that shoot Wither skulls. You can sneak while shooting this to turn it into a Howitzer instead.
The second item is a Witherite Block, which can be taken into a Crafting Table to turn it into 9 Witherite Ingots. Six of these are required to craft the Mechanical Fusion Anvil, used to upgrade items from Cataclysm.