When does Rimuru realize that Diablo is a primordial?

After a fun trip to have a concert in Lubelius

Then what happens ? Why is this such a big deal? I mean i know he’s very strong but what’s the significance of him being a primordial and under Rimuru?

Being a primordial means being one of the first 7 demons, so one of the oldest who gave "birth" to all of the others, and Rimuru managed to give him a name, showing how they are way more powerful than a primordial, which is a big deal and never happened before