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:

Despite my break down of the villains, I don't consider any of them to be inferiror to Kefka, I still feel Kuja is the best villain in the series and I actually love most of the FF villains except Seymour and Ultimecia (okay, so some of them are inferior but it has to do with certain...details...). So my above post is in no way a means of saying the rest are inferior as much as saying that they hardly struggled to get where they were; they are villains, they are suppose to spend most of the games being in control of everything. They are suppose to become absurdly overpowered with godlike powers from a silly magical McGuffin so we can have the kick-ass final dungeon and the epic final battle. Nothing wrong with that.