who is a good champ to play alongside Darius

I started playing league around 3 months ago and currently in silver. The main problem is I'm struggling to find someone to main I've basically played everyone in the game at least 3-5 time but have struggled to stick to one or two characters. I like characters with a good engage and good 1v1 potential any recommendations would be appreciated.

Who would you recommend to be in my champion pool along side karthus

I've started playing zed and am going to main him any recommendation on a 2nd champ to play along side him I was thinking kassadin not too sure tho.

I've only recently started playing league a month ago and have struggled to find a champion to main. The character I've mostly played is Shen who I've enjoyed but didn't like he's playstyle centered around helping teammates. What would be some champions that are fun to play have and have a good engage like Shen's E as I like champions with a strong engage thanks.