This is possibly the most alarming thing I've seen in my entire life. I have never shared my password with another user, but I immediately changed it when I got the notification. This happened in a Ranked Queue which dodged itself when I closed the pop-up message.
Has anyone else ever had this message, and what does it mean?
Riot support has been contacted, and I have changed all my passwords and traced sign-ins from all my e-mail accounts.