It's a very strong statement, indicating a complete and permanent separation. It's often used in situations where there's been a serious falling out or a decisive break.
Here are some examples of how it might be used:
* "Hasta nunca, te odio!" (Goodbye forever, I hate you!)
* "Después de todo lo que has hecho, hasta nunca." (After everything you've done, goodbye forever.)
While "hasta luego" is a more common and neutral farewell, "hasta nunca" carries a strong emotional weight and should be used with caution.