In Minecraft Forge, you can create a custom block by extending the Block class and overriding necessary methods. You can then register the block in your mod's registry. Here is an example:

​

```java

/* Custom block class */

public class CustomBlock extends Block {

    public CustomBlock(AbstractBlock.Properties props) {

        super(props);

    }

}

```

​

You can then register this block in your mod's registry:

​

```java

public static final RegistryObject<Block> CUSTOM_BLOCK = BLOCKS.register("custom_block",

            () -> new CustomBlock(Block.Properties.copy(Blocks.DIRT)));

```

​

For the datapack, you need to create a new folder in the `data` directory of your mod's resources. This folder should be named after your mod's modid. Inside this folder, you can create the following directories: `blocks`, `items`, `recipes`, `loot_tables`, and `tags`. 

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For example, the location of the blockstate file for your custom block would be `src/main/resources/data/<your_modid>/blockstates/custom_block.json`.

​

The block model file would be located at `src/main/resources/data/<your_modid>/models/block/custom_block.json`.

​

The item model file (if your block has an item form) would be located at `src/main/resources/data/<your_modid>/models/item/custom_block.json`.

​

The loot table file would be at `src/main/resources/data/<your_modid>/loot_tables/blocks/custom_block.json`.

​

The recipe file would be at `src/main/resources/data/<your_modid>/recipes/custom_block.json`.

​

For tags, you can add your block to existing tags or create new ones. The tags are located in `src/main/resources/data/<your_modid>/tags/blocks` and `src/main/resources/data/<your_modid>/tags/items`.

​

Remember to replace `<your_modid>` with your mod's actual modid.