Speaking of which I have to make an IMPORTANT ANNOUNCEMENT: THE BLACK SUN FORTRESS ON RALLTIIR IS BANNED AS IS THE TOMB OF FREEDON NADD ON DXUN.
Well, you wanted somewhere Xizor could go. He has countless other bases, including numberous space station on asteriods.But we don't have details because the Black Sun is TOO DANG GOOD at staying in the shadows. However, if you need another place he could go, it would be Fallen.
But seeing as that isn't a fortress but yet another planet (much less defended) I don't see why Xizor wouldn't just go mobile. His fleet of fast ships and pirate vessels would undoubtablly outrun the "millions" that Ulic's Krath Army has. Not to mention Xizor could use his intelligence network to predict Ulic/Kun's movements and conceal his own.