As Elidibus sent us back in time to Elpis, there was some critical realization that gave him peace in his final moments. Whatever that realization was is likely the point of the Pandaemonium raid series.

I think that if this story was primarily just about Lahabrea and Athena, then Elidibus' involvement in Pandaemonium would be superfluous, as Eric serves the same narrative function. I think we're going to encounter Azem, at least incognito, and get at least a partial explanation of what happened to them. And there's likely something in that event that shaped Elidibus' determination and made it deeply personal, even before he became the Heart of Zodiark. I think his promise was to protect the planet in Azem's stead.

With Pandaemonium now also existing in the present timeline, there's likely a threat to the star within Tartarus that has persisted into the present day. If I had to guess, I'd say it was the original Blasphemy. It's the only area that we hadn't cleansed yet in the role quests.