Work Items by severity report
One answer
You can find the BIRT report templates that are shipped with the product in the com.ibm.team.examples.reports.common folder of the RTC client installation (if you used Installation Manager to install the client, then you'll find this folder in the shared files directory). The template for Work Items by Priority is called "Work Items by Priority.rptdesign" - take a look at it to see if it can be customized to group by severity instead of priority.