Branded L'cie are given a "Focus". If they complete it they are turned to crystal. If they don't they are cursed to become cie'th.

I have to wonder if a similar concept existed in FF8. The Arch, the Gate they trap the Sorceress in during the parade, is based on the Arch of Titus, in Rome. Remembering Romans victory over Jerusalem, in 70AD.

What is the equivalent in FF8? Hard to say. The Arch was still there in Laguna's Past. So it's not Galbadia's victory over Timber. Although it would be interesting to connect the Iifa tree to Timber.

In FF11, the summoning art was "rediscovered" (as it usually is in the series lore) in Windurst, which was built on Kuluu ruins. So there there's another thread hear connecting Iifa Tree, Timber, and Diabolos.

In FF11 Diabolos is Terrestrial Avatar over dreams and dynamis. He appeared during the meltdown event that nearly killed life on the planet.
Although he's didn't "cull" everyone. He was just there to give the survivors an exit to another realm, if it looked like the planet wasn't going to recover.

Beneath Windurst "Heaven's Town" there is an underground lake/fountain sealing the Avatar Fenrir. The Summoner Job quests are kind of a mess, but are meant to be interwoven with the Main Storyline. I believe there was also a seal in Heaven's Tower that was sealing Diabolos. But it's easy to not question why Diabolos showed up when he did. He's been around much longer than Fenrir.