How do I create a custom validation rule with a custom error message in Laravel?

To create a custom validation rule, you'll need to create a class that extends `Illuminate\Support\Facades\Validator` or implement the `Validator` interface. Within your rule's logic, if validation fails, you should call `$fail('The custom error message.')` to trigger the error. Alternatively, you can create a custom rule class and then define the error message in the validation rules array when calling the `validate()` method.