Player Threatening me in private chat

So to abide by the rules here i will not reveal any names of players I played with or their characters to preserve anonymity, I just played a Nexus Blitz game where I played with a certain toxic player, which my entire team was getting irritated from, he fed, he had no contribution to the kills, so I muted him and reported him in the post game chat, while he was saying that he will never get banned because he is high elo and the game is dependent on people like him (i know, wth?). So he adds me, and usually i dont accept toxic requests, but I had positive experiences of cooling some people down and telling them it is just a game and some became in game friends, but this guy was actually threatening me. I took screen shots of what he sent me, but he basically said he can hack my IP, he said he can find where I live, and then he pretended to predict where I live (he said America) and I told you no you are wrong, so he said oh you are a foreigner then? and he started using racial slurs and other derrogatory terms to insult me, so i just blocked him. My question is, i reported him for his behaviour in the game, but how can I report him and attach the message evidence to show riot how he was threatening me? I dont feel safe playing around people like him and I just want to make sure he gets punished. Thank you [sg-zephyr]