Sephirot, also known as 'The Fiend", is one of three powerful Eikons. The other two being 'The Goddess' Sophia, and 'The Demon' Zurvan. These entities in the world of FFVI were known as the Gods of Magic, and responsible for the creation of Espers (our version of primals), and the existence of all magic in the world.

I can see how it is easily confused with FFVII's main antagonist Sephiroth, but they are indeed two separate beings. With Kefka making an appearance, even though it isn't really him, the actual Sephiroth making it into the game is a possibility.