When you defeat the Ixion Mark go talk to the petitioner who is the Whitecap Wench in the Seasand at the Balfonheim Port and she gives it to you as one of your rewards. I went through the whole game and just got it today after I forgot to go back to her. It would have made my battle with Hell Wyrm maybe a bit quicker; but I doubt that.

So yeah: Beat Ixion mark for Ragnarok.