There is no Excalibur used by player in VIII. Instead, if you enter the battle with Seifer at the end of Disc 3, Odin will appear (if you got him before). Seifer'll kill Odin, but Gilgamesh (of FF V) takes place. Like Odin, you can't summon him, he appears at the beginning of the battles (sometimes) and attack with one of his four swords. Excalibur is one of them.