A player did write in chat that he'll kill me with a knife irl: report is enough?

This. A report is enough in that it will have the automated system review the logs, and presumably will identify what was said as negative. However, even if that results in a punishment, it might still just end up being a chat restriction. If you can get a manual review to look over this, on the other hand, real life threats are among the few things that I have seen Riot issue instant permabans for.

If the OP feels that it was a legitimate threat in any way, the OP needs to be in contact with their local authorities first. Any in game punishment should be of no concern until the threat has been handled by the appropriate authorities.

That is something only the OP can decide but I just wanted to add that - if anyone ever fees they are the subject of legitimate or credible threats, they should forget about League and the other players account until such time as the threat is no longer credible.

[{quoted}](name=Tele II,realm=NA,application-id=ZGEFLEUQ,discussion-id=jsEwPaW0,comment-id=0006,timestamp=2020-02-24T23:57:49.458+0000)

Pretty sure someone went to jail for something similar. Theres an article i saw linked about it but i dont have the link, so youll have to google it if youre interested.

Was it the Runescape one?