PSA: Ranked isn't for trying new things out.
I get you're either intoxicated, you're just insanely toxic, you don't give a fuck about the game or your teammates, or you're just trying to "have fun", but that isn't for ranked. You have 3v3's, random game modes, Blind pick, Custom games, and Draft pick to try your Karthus Jungle in, not my ranked game. When I look at your op.gg, and it says that you have a 24% win rate on Karthus Jungle, I'm going to dodge. I don't care if that's breaking the rules, I'm not going to suffer -18 LP, 20 minutes of wasted time and just an all around headache because you refuse to pick something that's viable. "I don't get why the Kayn jungle is allowed to counter jungle me all game", gee, I wonder why. It's probably because you can't even first clear your jungle without having to back and can't get a single successful gank off.
That's my rant of the day. Stop being selfish and play something that you're familiar playing. If you're a Karthus main jungle with 200 games, with an over 50% winrate, then go for it. Don't just lock that shit in because you're in discord with your duo buddy having a good time. Take consideration for the rest of your team.