I am playing the Draconic Racial Class (Races of the Dragon pg. 70) and would like to take the Necropolitan template (Libris Mortis pg. 114). May I do so in between the second and third levels of the racial class (when you take the second level you are required to take the third as your next level)? Does this result in a half-dragon necropolitan?
Yes and Yes-sort-of
Necropolitan is a template that can be added to any Humanoid or Monstrous Humandoid, and the draconic racial class does not change your type to Dragon until third level. Moreover, because Necropolitan has LA +0, it cannot constitute “your next level” for the requirement that the third level of Draconic immediately follow the second. In fact, Necropolitan involves losing a level, which means you actually lose the second level of Draconic. You may still take it when you regain the level, and then the level following that must be the final level, completing the Half-dragon template.
At the end of this process, you will be a Half-dragon Necropolitan X, where X is your base race. Your type and subtype will be Dragon (Augmented Undead, Augmented Humanoid [or Monstrous Humanoid]), you will have all the benefits of the Half-dragon template, as given to you by the Draconic racial class, and you will have the special qualities of the Necropolitan template.
Note, however, that Necropolitan does exceptionally little for you excepting that it turns your type to Undead. When you then gain the final level of Draconic, your type becomes Dragon, eliminating the benefits of the Undead type. Thus, you are losing a level for just these things:
This strikes me as very much not worth it. Unfortunately, you cannot reverse the order to get an Undead (Augmented Dragon, Augmented Humanoid) creature, because Necropolitan cannot be applied to creatures with the Dragon type.
Also note that your DM is very likely to houserule this situation. The fact that you could not apply Half-dragon to a Necropolitan (“‘Half-dragon’ is an inherited template that can be added to any living, corporeal creature”) and you could not apply Necropolitan to a Half-dragon (“‘Necropolitan’ is an acquired template that can be added to any humanoid or monstrous humanoid”) is a very good reason for a DM to nix this combination. The only reason it works because you apply Necropolitan halfway through applying Half-dragon using a sidebar variant that failed to specifically spell out that it can only be applied to a creature that you can apply Half-dragon to. If he does allow it, I would expect him to houserule some of the interaction because it doesn’t really make sense for you to undergo the Ritual of Crucimigration and become undead, only to somehow come back to life as the Dragon type (though frankly a pretty freaking awesome plot along those lines could definitely be possible).
All that said, I really would strongly suggest using a different method for becoming “dragony.” Draconic is kind of meh and Half-dragon is quite weak. Personally, I strongly suggest that you just take levels in the Dragonfire Adept class from Dragon Magic, it does a very good job of allowing you to have dragon traits (Draconic subtype, breath weapon, flight, etc) without the problems that the LA system causes.