High elo Karthus

I used to be a main Karthus back in my gold elo, with about 80% win-rate out of 100 games. However, I stopped playing him and I got Diamond with other mains, next season. Once I played some games again with Karthus I feel like he's so freaking useless and countered with almost any champion. Are here any high-elo Karthus mains to tell me his pros/cons and the team composition he's good in ? Do you think he needs a buff ?