I hope this message finds you well, and congratulations on your clothing line. I just performed a quick search of the mark and there is no active (or dead) trademark for royalty kings. That being said, that is not the only test as to whether you are in the clear or not.
The test of trademark infringement is more or less predicated on a "likelihood of confusion" by the consumer. Would consumers be confused as to what company or line of clothing this is and who produces it?
Based on name alone, the answer would be that there is a substantial likelihood of confusion, and you would be infringing this person's common law right to the mark (simply by using the mark in commerce, a person has a common law right to the mark defensible in state court).
That said, if you were to file for trademark protection and this person did not protest the filing, you would likely receive a federal trademark. This would give you the presumption in federal court that you are the lawful owner of the mark. It would be up to the other person to show the court that they owned and used this name first in commerce and therefore you should be barred from use.
This may be a presumption that they could overcome with proper documentation. Moreover, you would have a very limited ability to sue them for infringement because they could use this defense to their advantage. To that point, if they obtained any level of success, or if you are successful, having the same name in commerce could be a business liability for you. It would also undermine the value of your brand if you were to try to sell the clothing line to another business.
In summary, if you use this name, you could technically be infringing on their common law rights. You could file for and potentially receive a trademark, but it would be a weak mark that could be undermined by this individual. Moreover, the business model would be weakened as well. I would caution against use of the name Royalty Kings and develop another name that is not directly or indirectly in the line of fire for infringement proceedings. I think that will not only be the better legal move for you, it will also be the better business move (which is what it is all about anyway).
I wish I had better news for you, but I am obligated to convey the legal reality of the situation. Let me know if you have any other questions or comments.
Best wishes going forward!