Validation Rule on Roll-Up Summary Field / Custom Object

I'm looking to create a validation rule on the opportunity object. I have a roll-up summary that is pointed to a custom object that is related to the opportunity by master-detail.

I'm trying to fire the validation rule when the opportunity stage is changed to stage "X". I want to ensure that there is either at least one row in the custom object related to this opportunity or that the roll up summary field is > 0. Either one of these conditions would work for our use case.