For all we know, the ancient order of wizards used up almost all of their resources dealing with Ythog-Nthlei's brethren over the preceding 200 years, and by the time they reached the last of the "sons of the old ones", there were so few of them left, and their spells/scrolls so nearly exhausted that all they could manage to do was seal Ythog-Nthlei in this dungeon.
Who knows exactly what happened? My point is that I'm not sure that such speculation is terribly helpful for the purposes of a conversion!
I feel vaguely guilty disagreeing with that decision given that I haven't been involved in this conversion at all until Freyar cast that summoning spell a few posts back. But only vaguely, and I do disagree with it
The whole purpose of the Creature Catalog -- at least as I understand it -- is to present 3rd edition conversions of creatures from earlier editions. In my mind, that means that as a general rule, the conversions should try to stick relatively closely to the original creature, and not present versions of old creatures which are radically more powerful than, or substantially different to, the originals. Otherwise they are not really
conversions.
I don't have a problem with the "Beyond Time and Space" ability, or with very powerful epic-level creatures. My campaigns have historically tended towards quite high-level play, and this version of Ythog-Nthlei would be a handy addition for that sort of campaign.
But it seems to me that it is already quite a stretch of the Creature Catalog's original intent to present an epic-level Ythog-Nthlei as a "conversion" of the one in Dragon #46. I think that adding an additional side-bar with abilities which make Ythog-Nthlei even more powerful stretches the nature of a "conversion" so far that I'd be inclined to say it no longer belongs in the Creature Catalog, and should instead be something for homebrews.
Sure, it doesn't hurt to give a DM extra options. Speaking personally, I think I'd be more likely to use an unimprisonable epic-level Ythog-Nthlei than a CR 14-16 version. But does that mean I think that the more powerful version really belongs in the Creature Catalog? No, I'm afraid not.
Sorry Cleon -- I hope that doesn't come across as too deflating a reply. I really dig your creative monster design, but I'm afraid that I'm a bit more of a purist when it comes to conversions of older creatures.