Riot's ability to intervene would (I imagine) pretty much be limited to taking it as a complaint and suspending or banning the account of the aggressor, but the question of something like "Police intervention" is outside of their jurisdiction. Riot Games is a private corporation, they aren't law enforcement; whether or not police could or would take any action would completely depend on the police agency in question.
In this hypothetical situation, the 'victim' of the threat would need to contact their local police department and determine how they handle these sort of statements. There are legal ways of obtaining someone's address (and plenty of illegal ones), but simply saying "I know where you live" may or may not be considered a criminal act depending on the location.
And then it would depend on the circumstances of the threats and the police agencies involved - if the aggressor and the victim are in different countries (a very real possibility on an international game like League), then the aggressor may be outside of the police agency's jurisdiction to take any kind of intervention. Unless someone actually showed up at the house of the 'victim' I doubt anything would come of it.
This is speaking as a police & ems dispatcher. In our state, this likely wouldn't qualify as Harassment, and even if it did, likely would not get past the DA's office to prosecution.