The simple way to achieve this task is to "Self-Join"
I would self-join the data again with Inner join on Region and Era
![something like this](https://i.stack.imgur.com/1Rs80.png)
then, to handle the duplicate rows I would create a calculation
[Empire_Data1] = [Empires_Data2]
and put as false in the filter shelf.
then if you drag both Empires field you will the output you are looking for,
![In my data, I'm calling "Empire_data2" as Relevant Empire](https://i.stack.imgur.com/ZbUKi.png)
Since this is like 20 rows of data, you can perform a self join without any challenge.
But you have a lot of rows as in hundreds of thousands or more then, you might want to prep your data before connecting to the tableau.
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