Need to trigger a notification when all the tasks associated with case closed complete?

Hi All,

I have a requirement that , basically when HR case is created , they manually create HR tasks, i want to trigger a notification that it checks all the tasks associated with case as closed complete then the notification will trigger to health group? and that includes body as 

Hi <HR service name>

Please note that the tasks you have requested on <date of tasks submitted> for <Employee Name> have now been marked as complete.

Please help to resolve this

Thanks

Ramu