Cross Object Validation Rules

Hi All,

I'm hoping to make one of our custom objects a required field based on the records within a 'standard' object. I've previously never set-up a similar process between to objects so wondering if this would be possible?  

We hope to set-up a process / validation rule that would prevent users from closing cases if the Case Record Type & Case Type match the criteria, which would then force users to complete all required fields within the 'custom object'.   I believe this would involve a cross-object validation rule as we'd like to force all cases (object 1) that match the criteria, to force the form to be filled on the 'custom object' (object 2). Would this be possible? If so, what would be the best way to approach this?  Look forward to hearing back from you!

Thanks in advance,

MD