Just found a nice little discussion on /a/ and thought that there hasn't really been any reconciliations about their exact nature other than as the designated bad guy.

I don't think there will be a concrete explanation as to their origins, but in the SWverse history, the fates of humanity and Neuroi have always been intertwined. In the setting, any things that would've been fantastical in our world (Other than magic itself of course) is attributed to Neuroi.

Ancient Youkai? Neuroi.
Medieval Dragons? Neuroi
Demons and Monsters? Neuroi.

It seems that Humanity and Neuroi were just destined to be enemies with each other as their conflict has persisted throughout history. If we were to pin a particular motive for Neuroi, then it would be because of the fact that they gain sustenance from devouring land. (Hence the missing landmasses in the SW world; they were casualties from Neuroi conflicts spread throughout history)

It seems that there followed a period of peace in this age old conflict. Due to the relative lack of major destruction alluded to in the setting's recent history and general lack of knowledge of humanity about Neuroi. However, during the onset of the industrial revolution and the dawn of magical technology, as if in response to Humanity's newfound technological knowledge, the Neuroi themselves awoke again and evolved from fantastical creatures to the more mechanized appearances they have today.

The short skirmishes of humanity against the reawakened Neuroi (Then known as the Kaii in the east or Ratsel in the west) later became retroactively known as the 1st Neuroi War. Further smaller Neuroi conflicts also occured in thr next few years. (i.e. the Fuso Sea Incident) Around the late 1930's the Neuroi began an assault out of the Black Sea en masse, causing heavy casualties and losses in the European region. Their emergence from the Black Sea also gave them their current Moniker: Neuroi, in relation to the legendary tribe that allegedly lived in the area in ancient times. This started the 2nd Neuroi war, which is the current setting of the SW series.

Other than Neuroi eat land, we don't really know anything about the Neuroi's ultimate objective? Eradication of humans? Complete cannibalization of the earth?

One interesting thing an anon pointed out was that while Neuroi and Humanity have indeed clashed a few times in history, this was the first time a Humanity vs. Neuroi conflict grew to such a massive scale. In fact, the Neuroi only became heavily aggressive during the industrial boom when the first magic engine was created. This raises some interesting implications.