What is the special tribunal for Ukraine that world leaders have backed?

Description

This is an explainer article about the Crime of Aggression and Ukraine. I was asked to comment for it. 

Same piece is then republished in other places (because the AP is an agency), such as the Washington Post: https://www.washingtonpost.com/world/2025/05/09/ukraine-aggression-tribunal-europe-russia-war-court/a8720174-2cdd-11f0-a724-3bc879c9f843_story.html