Hello,
Congrats! There are only a few candidates invited to the Assessment Center (AC) (around 8-10). So good job so far!
The AC is going be a "full day screen" so try to get a good night sleep the day before. As it takes some hours, I can only recommend to be authentic and to try to be yourself. There will be role plays and a case study to solve, but nothing really bad. However, I don't know the questions as my AC was 6 years ago...The most important thing they want to see is how you react in stressful situations, how you behave in a team and how you solve problems.
Good luck!
Sabine