There are several different reasons that your homebrew could be rejected on copyright grounds:
- The name or description matches an existing Wizards of the Coast intellectual property (mind flayers, beholders, mordenkainen etc);
- The name or description matches a copyright registered by another company (Darth Vader, Captain America, Pizza Hut);
- The image you have used is artwork that is copyright protected - don't just swipe the first image you find on google - check the usage rights first.