If Xykon desperately wanted the Orb of Super Awesomeness, he would fly in and take it, eradicating anyone who annoyed him and vanishing before the authorities arrived. Maybe bring a few zombie ogres to smash the anti-magic crystals/walls/shoebox if that's an added obstacle.
The only reason Xykon needed the army in the first place is because it was not a prize he could just fly in and swipe. As Fyraltari said, (morally questionable) epic-level arcane casters don't take jobs. If they want something, they take it. If taking it requires an army for whatever reason, they hire (or conscript/enslave/conjure/create) the army, not vice versa.