I believe people were asking for champion with Scythe for a while
http://forums.na.leagueoflegends.com/board/showthread.php?t=3105009
4 years before u
http://forums.na.leagueoflegends.com/board/showthread.php?t=2913677
5 years before u
If u just search u could find tons of threads asking for it
This cannot even be your idea. You stated nothing of what you wanted the champion to be in the first place. You even said as a comment / reply to another person that you "don't want the character to be about the scythe" when Kayn clearly is all about his scythe.
You just said "I think a champ with a scythe would be cool". You cannot claim a champion that has a scythe is stealing that idea.
well I created a scythe wielding champ based on shadows and :P nothing https://boards.na.leagueoflegends.com/en/c/skin-champion-concepts/HKO7WAab-champion-concept-please-review-riot
I made a Scythe Wielding champion aswell that had a dash and a passive about collecting something, and yet i do not complain because i am just happy they did it.
They stole nothing until you can show proof that your idea had Copyrights which were broken, which everyone can already agree on you don't have.
Theft only becomes a claim if what was created/used was taken from someone who officially, rightfully and lawfully owns it.