Banishing/Sending is not a choice. If you have a DARK in hand, you must banish it. If you don't, you must send your entire hand to the GY.
If "Imperial Iron Wall" is chained to this card: If you have a DARK in hand, you must reveal it, but is isn't banished. If you don't, you must send your entire hand to the GY.
You can't activate this card if "Imperial Iron Wall" is applying.