I first interviewed with Riot a year previously for a QA position (requiring an analytical aptitude test, several phone interviews, and an on-site interview): they quickly and accurately identified that my passion lay in Game Design instead, and recommended that I reapply in the future for a Designer role.
For this interview they actually contacted me with an interview offer, and I underwent a total of a two-month-long series of design tests and phone calls (two design tests and three or four hour-long phone interviews), ending in an invitation for a second on-site interview.
I rank the interviews as "extremely difficult" only because they were intentionally designed to challenge and push your ability to perform analytical and creative design under pressure. If you find them anything less than difficult I'm almost positive that you're not diving deeply enough into the design decisions you're asked to make -- or you have much more industry experience and are therefore applying for a higher level position than I did.
Although one or two of the interviewers seemed like they were attempting to overwhelm me the overall experience was incredibly invigorating, and it's clear that Riot has carefully constructed the interview process to weed out people who don't have the skills necessary to succeed in their company environment (both social and professional). There were opportunities to push back or challenge the interviewers, and it seemed like some of these opportunities were to test your ability to defend your design choices, while others were intended to see if you were willing to accept criticism and back away from a poor idea. 11/10 for the interviewers -- I left feeling mentally drained yet energized at the same time.
Be ready for some questions that seem strange: Riot likes to test you thoroughly, and they may judge your analytical or design skills in ways that seem highly unorthodox. I'd also recommend being completely honest with them -- they'll pick up on your strengths and weaknesses very quickly, so don't fake passion in something you're not interested in or gloss over a shortcoming. Embrace it, talk about how you're working on overcoming it, and show that you're a candidate worth investing time in.
I didn't land the job, but if you're a good candidate Riot is ready to invest in you: I received a surprising amount of information about why I didn't make the cut, how close I was, and exactly how much everyone liked me, along with some really helpful information on what to improve upon, and a request that I consider reapplying in a year's time for a similar position.
Overall a fantastic experience at an amazing company. Everyone was incredibly polite and friendly, very passionate, and obviously extremely intelligent. It definitely convinced me that Riot is a company worth reapplying to.