The first time I played FFX, I knew I'd heard this track before. At first I thought it was earlier in the game, but I couldn't for the life of me recall which track it was. Years, YEARS later, while replaying FFIX, I finally remembered. It's from the track "Assault of the White Dragons" from FFIX.
I'm not the biggest fan of rock, but as The Black Mages had Mr. Uematsu in and were doing the amazing battle themes to the FF series, I had to give it a listen... and this one definitely one of my favourite tracks! That guitar solo at the end is absolutely stunning!