body: Suppose that I made an MMORPG which has interesting gameplay, but rather bad graphics.
  
  
  One day, a fan of the game writes a client in Unreal Engine that lets them play the game with HD graphics (but they still need to buy subscription from me to connect to the server). Because of this, my game becomes more popular and I'm making twice as much money.
  
  
  Epic's license for Unreal Engine asks for 5% royalties, but I never agreed to this licence, the person who made the third-party client did.
  
  
  Do I have to pay royalties to Epic? If yes, is it just on the portion of profits from players that connect using the third-party client?
  
  
  
title: Third party client made in Unreal Engine