Kefka

[NeoCracker]

He stumbles upon it cause he first encounters it when Mog transforms into Madeen. Its only after he realizes how strong Trance is and decided to leave for Terra with a plan.

When the battle with Garland and Kuja commences in Pandemonium, Kuja reveals he's used the strong emotions from all the souls captured by the Invincible to allow him to enter Trance cause he's actually incapable of doing it on his own. He makes a comment to Dagger about Brahne's wicked feelings being a part of what gave him power. The Iifa Tree really had nothing to do with it. :vivi:

I recal the Iifa tree being a big part of the soul gathering, as well as the Invincible, though it's really not to relevent to the discussion. :P

And I thought he had seen trance before, I figured that whole scene was him getting the Idea of just how to release a trance himself, rather then him stumbling across it, much like Kefka with the Triad.

Though yeah, I guess one could call that stumbling if one desires, it's all semantics anyway. :P

[Wolf Kanno]

The Iifa tree is not really involved with Kuja's plans, it deals with soul manipulation but that has more to do with Garland and Terra than Kuja.

You are right about Kuja knowing about Trance before the scene with Mog though. He does mention Trance with some familiarity though I can't seem to find out where he learned about it from the script. In the scene in question, Kuja finally figures out how Trance works.

Kuja: "So...an eruption of anger against one's surroundings induces a
complete Trance! It's not the will to live, nor is it the desire
to protect another!"

Kuja: "That means...I simply need a powerful soul, even if that soul
is not my own! No, wait... And it can be found there! Hahahaha!!"

The script and other sources pretty much states he finds these strong souls in the Invincible.