Adding this follow-up in a separate post. This is condensed from the module itself:
He went to the wizard and told him how he had once summoned the demon and imprisoned it in a binding circle. He thought he could do so again and force it to save the city, but while he had the true name and symbol of the demon still preserved, the circle of binding could not be drawn on the rough terrain inside the city... <snip> The wizard, however, did know how to create a stable binding circle, one that would last for centuries.
the module does not go into any game mechanics/specifics, other than the wizard spending many days cutting & laying out gems in a precise circular pattern, all placed in specific locations and each engraved with runic symbols.
Well, there's the "DM handwaving" for how the Marilith stayed trapped for centuries. And while the flavor text may say "summoned", and indeed most players, characters, and NPCs are more likely to use the word "summoned" than "called" to describe the circumstance, the rules would classify it as a "calling". Greater Planar Binding (Sor/Wiz 8) would work for a Marilith. Not that I expect the module writers were too concerned about the strict rules when writing background flavor text.
So, in closing:
1: It's background flavor text, so you can DM it however you want.
2: If you like to have things work according to the Rules As Written when possible, then the Marilith should be a Called creature, and hence *can* use its Summon ability (as well as its teleportation, which monsters summoned by a Summon Monster spell can't use, although even Summon Monster IX can't get you a Marilith; the non-teleport clause doesn't seem to apply to monsters summoned by the Summon special ability, however). It's got at best a 50% chance to Summon something, though, so feel free to fudge the result to avoid a TPK.
3: If it's a Called creature, then (by RAW) it will really die if slain; if it wants to get back "home" (in a non-afterlife sort of way) it'll either need to find some means of dimensional travel; someone willing and able to banish it (e.g. using Dismissal, Banishment, Dispel Evil, Holy Word, etc.); or (if possible, and depending on the spell) fulfill the task for which it was Called.
4: If, instead, it's somehow Summoned, then it can get back home (with minor inconvenience) by "dying", and it can't Summon (or Teleport, if it was Summoned by some sort of Epic Summon Monster spell). It will also presumably disappear relatively soon in any case when the Summoning wears off.
5: Another very important distinction: Summoned creatures can't penetrate Protection from Evil (etc.) and Magic Circle against Evil (etc.) spells (though their weapons can). Called creatures suffer no such restriction.
I hope this helps. In the end, play it how you want, and have fun!
