Let's go over the equation for Xykon's Magic Item That Grants Him Fire Immunity, shall we?

Energy Immunity is a 7th level Sorcerer spell, so that gives us a 14th-level minimum on caster level, as it was created by Xykon himself. A continuous effect with a 24-hour duration would be (spell level X caster level X 1,000) GPs, giving us a market price of 98,000 GPs(keep in mind that, if you are planning on having this for yourself, the price will be lower if made by a Wizard, and even lower if made by a Cleric or Druid). Although this would take 98 weeks to make (~2 years), he has had more than plenty of time on his hands to do so (unless if he took the Epic Level feat that would decimate this time, requiring only 10 weeks of time).

So there you have it. He possesses a magic item (most likely a Wonderous Item, as he already has that feat) that required 49,000 GPs in materials, and 3920 XP to craft. Boom. No absurd Epic rules needed. Just Complete Arcane (page 105) and the Dungeon Master's Guide (page 285).