I kind of think of the language as sort of the umbrella force or expression of a given sphere (of the 6 Prime spheres in the cosmological chart). Necromancy could be said to be an expression of Domination in the same way Pyromancy is an expression of the Arcane, or healing magic is an expression of the Light. The difference between a branch and the roots of a given Sphere, you could say.
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Domination has always been a part of Necromancy, though; in that, a Necromancer raises the undead as his or her thralls - bound to their will, which is what Domination is all about. True Domination doesn't limit itself to the undead, though; it is more about dictating terms to reality itself and can hold much more in its thrall than simple servants. It can, if wielded by a master such as the Primus, bind even god-like beings in the same fashion that Zovaal himself was bound, at least until he managed to free himself eons later.
As for the whispered utterances of the Everliving Statuette, it's really too early to say if these have any bearing on the First Ones - assuming they're not outright lies from an object with a noted penchant for duplicity. That being said, this supposed "Seventh" that was either cast out or suppressed in the design of the First Ones probably stands apart from any of the Spheres we currently know, including Death. Domination is part of the Sphere of Death and was cultivated by one of its Eternal Ones directly - it doesn't stand apart from the design of the First Ones, but is explicitly part of the system they created and was a direct creation of one of their direct creations (the Primus). Whatever the Seventh is, if it's actually anything, is likely different from any of the six known Spheres or their attendant essences.