I suggest you take a slightly different path than putting that in CV. Putting a gaming achievement could be an instant turn off for many recruiters especially if they do not play video games. Especially if they have kids playing lots of video games. And let's be clear about this, they don't care about your boss kills.
What do they care about? Well depends what position you are after. Are you applying for a position where you'd work alone maintaining some IT services? I'm not sure they put much price on leadership skills. Are you applying on a position where you'll be part of a team expected to take responsibility and step up? Suddenly your experience is relevant.
What you write and how you write it should be tailored on the position you're applying. What are the skills the company wants to find in the candidates and how can this experience be relevant for them so you can demonstrate you're up for the challenge. Gets even more valuable if you are after positions in gaming industry. A lot more valuable.
I would personally mention this in the CV as a relevant hobby and won't go into much details there. Stick to facts and number: number of people, timespan, major ranking or achievements. Short but powerful. If someone not knowing you is reading that part they should instantly say How? Where? Give me more details! For the rest of the CV I wouldn't want anyone to think of me as someone that plays video games all day long and believe I am not capable of working so I want to make sure in the CV that I am a match for their profile. I would stick with things they required explicitly and are relevant. Schools, courses, side projects, small jobs and so on.
The letter of intent is the place where I would go a bit more in details. And yes, every application I send has a letter of intent. From my letter of intent, which I already know it will be read after my CV, I want to back up my CV with clear experiences but also demonstrate how I am over other applicants. And here is where references to my skills obtained in the virtual world go. But I would still be sure to not make those the highlight of my experience. I would exploit the diversity, the difficulty of coordination over the internet and to some extent the leadership capabilities. Aim, where possible, to back it up with other experiences as well. I am supposed to work with people from multiple countries? Well here is what I did before and here are the results of that. Happens to be in a video game, nevertheless, the same skill can be easily identified in both examples.
The interview, this is another story; I already have a much more flexibility and I can work with the interviewer to ensure my experience is not misunderstood. Here I can briefly go over difficulties encountered and how did I used my skills to solve them. It is easy to specify that even in a virtual environment the responsibilities and skills are very similar with any other project. I could demonstrate some of them and even go together over some important past example. Maybe how you solved a major conflict or how did you deal with an especially important problem. And always translate that to their job. You coordinated a group of 25 people and got a realm first kill (or whatever achievements that game has)? First find out whether such a situation exists in their company. There are cases where short term coordination would be need? If yes, go into the details on how you did that in the past. Lastly come back to their situations and show exactly how that would translate nicely for their need. BAM you solved the company need, you bring value, you are suddenly relevant.
Example:
"You: There are people in the team working remotely?
Recruiter: Yes, our company is present in 8 countries and often collaboration between different people on different time zones is needed?
Y: It happens to face issues because of this? or Would you say it is important for a future employee to be able to handle such tasks / communications?
R: Yes, for us this is vastly important [bla bla]
Y: I can understand why this is such important. As it happens during the past 5 years [your experience]. Such skills could definitely be handy here because [how you can handle their problem]. I am glad to see the job presents an opportunity for me to further polish my [targeted skills]"
Bring value. As long as you bring value, any skill or experience is relevant.
Good luck!
