And the answer is: necropoleis or necropoles. The first form comes from Greek, where the word originated (literal meaning: city of the dead), and is pronounced as if it were spelled "-ase." The second form comes to us through Latin (as most of our Greek-derived words do), and is traditionally pronounced by English speakers as if it were spelled "-ese" (although in Latin it would have been pronounced as "-ase" again). Necropolises is also perfectly acceptable as an English-native pluralization, so don't be too hard on your fellow adventurers who just want directions to the darn things.
There are a number of words that are not a reasonable plural of "necropolis." Foremost among them is "necropoli," which arises by mistaken analogy to words like "cactus." Although the -is ending in "necropolis" is pronounced more or less the same (depending on your dialect of English) as the -us ending in "cactus," it does not come from the same category of Latin words (third declension as opposed to second, if I'm not mistaken), and thus does not pluralize the same way. Just say no to "necropoli" -- kill that urge and fight the Scourge.
Edit: Yes, I know that languages evolve, and that M-W likes "necropoli." I still don't like it, though, mostly because it's based on a bad analogy. Now get off my lawn, you crazy kids!