For what it's worth, I don't think Elidibus was out of character here at all.
I mean yeah Elidibus is DIFFERENT here than he was before, but I would argue the change is justified. Consider this: For thousands upon thousands of years, you have been able to control who and when new heroes rise up to challenge your friends, and you have done so in service to a specific duty related to maintaining balance between light and dark. One day, you feel that dark is too powerful again, so you approach a random adventurer who gave one of your friends some trouble and test them in order to determine their fitness as a new hero.
From then on, that random adventurer goes on to become a full-fledged Warrior of Light and accomplish many great deeds, but you're always confident in your ability to control the situation if needed. That is, until they are whisked away from certain doom and transported to the site where the next rejoining is in progress. From there, they go on to stop the plan and even murder your only remaining unsundered comrade-in-arms.
Now, suddenly there is something very different about this random adventurer. Suddenly, they've demonstrated abilities faaaarrrrrr beyond anything you've ever expected. Suddenly, you're not in control anymore - You may not even be able to defeat them if you tried. Now you're desperate, scared, trying to find something - Anything - That will let you gain control back over the situation. That's definitely going to make you drop the "hur hur I'm a big bad brilliant schemer who knows all" act and very rapidly too.