This was a very long process. I went through 7 phone interviews before I was invited on-site. There was some confusion as to what role was best suited for me since I had a wide array of skills. The interview process seems very unstructured and each interviewer sort of "wings it" and asks what they want. None of the phone interviews are particularly challenging but the interviewers put a lot of emphasis on what you intuitively know.
The on-site interview is a mix of behavioral and technical. The technical questions weren't very difficult and the interviewers simply want to get a handle on what you've done previously and if the experience is translatable. The main focus is a "pitch" you have to do for two Directors at Riot. You will be given little guidance and simply told to pitch a product vision and how you would execute upon that vision. They seem to be in a phase where their preference is for seasoned veterans in the gaming industry and if you're coming from a different industry, it would behoove you to do some additional research. There are no bonus points given for asking the right questions. Also, I would advise against pitching anything too specific because it will open you up to conflicts with their internal initiatives. If possible, try and learn what these are before you go on-site. Finally, get a good sense of what their staffing is like in terms of engineering capabilities and personnel allocation. You will be asked specifically what sort of team you will need. Looking back, I would have asked previous interviewers these types of questions in the Q&A.
My biggest mistake was picking a product idea that did not mesh with current initiatives, was mired in technical detail (due to the nature of the product), and not having a detailed plan (because I came in with too many questions about how Riot prioritizes, what other projects are in pipeline, internal metrics of the problem, resource allocation, etc.).