Trance

The trance system was a new system introduced to FFIX. The basic idea behind it was that when a lot of emotion (anger) built up in a character, their power would temporarily increase. At first this doesn't apply to Kuja, but he eventually figures it out.

In Mt. Gulug as Zorn and Thorn try to extract an eidolon from Eiko, her moogle protects her. It ends up going into a trance, and that's where Kuja figures out a few things. "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! 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!!" Kuja realizes that he can experience the same powerful effect by using other's souls, and he doesn't waste much time in doing so.

Once in Terra, Kuja undergoes a complete physical transformation. He takes on a red and feathery appearance in which his tail is fully exposed. This is his trance form. He explains to Zidane and the group that he used the souls that the Invincible had sucked up to create his trance. One soul was not enough for him -- he needed many to take on Zidane's group. He even goes as far as to say that he is using Garnet's mother's soul. Kuja from that point on is unmistakably more powerful.