Link three tables to on Foreign key

From this ERD could anyone tell me what the best way to connect the Modifier, Monster, and Equipment tables to the card table. I cannot just give have those tables have a FK of CardID because a Card may be the same Card, but have a different illustration, which would make the Card Unique but not the ModifierID, MonsterID, or EquipmentID. And I don't think you can have null foreign keys either. Is there any other way besides three more many to many tables?