"Necromantic" is the adjective form of the noun "Necromancy", with "necromantic magic" just meaning "magic that is Necromancy".
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At the time it made sense as Chronicle was supposed to be the primary source of truth for WoW (at least, that is what is was marketed as). It doesn't help that Grimoire of the Shadowlands provides us a significantly more basic cosmological chart with emphasizes Life and Death over every other force and removes the association between the various forces (i.e.: Disorder, Order, Life, Death, etc.) and the magics associated with them (i.e.: Fel, Arcane, Nature, Necromancy, etc.). We've been put in a situation in which a random Broker knows more about the cosmos than the Pantheon of Order, and possibly the Pantheon of Death. It could be argued that the respective Pantheons might just not care too much about the other forces, which may be why the Titans were largely ignorant of many facets of Death, but it just seems weird for them to have less insight.