How can I set up a validation so that the Core.Description has to be unique across all node

Summary:

How can I set up a validation so that the Core.Description has to be unique across all node and node types in a given Viewpoint

Content (required):

We currently have the Department Structure in EDM. I am finding that the system is allowing for departments to have the same description. I would like to set up a validation, similar to the Core.Name where the Description has to be unique. I have included a snapshot of the validation I want to set up. However, I can not as it is indicating that the when using ancestors or siblings the equal to must be a string.