The were impostors in the sense that they were not of "The Many". There undead state was not caused by the oaths that bind "The Many". They were undead, they could raise undead but they are completely different. What is wrong with this, why should there be only one source of undead? So far as I understood it the necromancers could not control the undead of "The Many" they created their own undead forces. Thus the label impostors.
Would it not be prudent to notice that "undead", "monster", and possibly "daimon" too are generic terms which may apply to different kinds of them? There may cover both different societies of creatures or societies of different kinds of creatures, today dominated by some, tomorrow by others. Say the third invasion was done by skeletons and the fourth by zombies, for example.
