Final answer:
The banner in the Hall of Evil proclaimed that evil is the nature of mankind and should be embraced as the only source of happiness.
Step-by-step explanation:
The banner in the Hall of Evil said, "Depending upon one another's hearts, ye had still hoped that virtue were not all a dream. Now are ye undeceived. Evil is the nature of mankind. Evil must be your only happiness. Welcome again, my children, to the communion of your race." This quote emphasizes the belief that evil is inherent in human nature and that it should be embraced as the only source of happiness. It serves as a warning to those who cling to the idea of goodness and virtue.