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The prologue merely states that it was due to their misuse of such powers that they had been banished long ago. Their is referring to the Voltaris, and such powers presumably meaning the prime songs. However, if we take into account that the Ardoni seem capable of magic other than the songs, such as the time portal, or possibly Ingressus color changing, then necromancy might be another available branch.
The Deathsinger allied with the Nether. If the clan has already had experience with the undead, and there was some history there, that would explain how the alliance formed so quickly. Also, the name Deathsinger could be a hint.
We dont see any undead Ardoni. Possibly instead of becoming undead, the Ardoni are in fact resurrected. There may be some very specific requirements, and could be the reason why Ardoni females are so rare. The Voltaris seem to have more female Ardoni than the other clans. Perhaps these requirements arent met now, or the Voltaris mostly stopped practicing necromancy in exile.
The Badlands, overrun with the undead, are relatively close to Old Voltaria. The badlands could be a remnant of the Great War, or it could be a result of the Voltaris having experimented with necromancy long ago.
Building on the above theory, the Necrolord is part of the larger plan to throw Ardonia into chaos right when the Deathsinger would return. The Necrolord has glowing red eyes reminiscent of the Voltaris and seems to be slightly taller than the other Necromancers.
The necromancers also light Niika's tree fort on fire. While this could be considered a standard tactic to destroy enemy information, burning down places seems the way Voltaris operate- they burned down Senn's village in Sendaria and presumably the other Ardoni villages they attacked to get the prime songs. The Deathsinger carries a fire sword and both in the prologue and in the fight at the end is shown in burning areas, some of which he likely set on fire himself.
We're shown Thalleous experienced negative effects due to a "lifetime of wielding songs." However, why would the Ardoni wield songs so freely, and why would Thalleous give both Senn and Ria songs if they were so dangerous to use? Of course, it could be argued that the benefits of songs outweigh the risk, but there's another possibility; the Prime songs are uniquely dangerous. Whatever negative consequences songs have are amplified with the Primes.
In the duel between the Deathsinger and Tidesinger, we learn that anyone who uses a Prime song seems to be fundamentally changed somehow. We also see that the Prime songs hold the attention of anyone who sees them, not just Ardoni. Both Senn and Sulliman can't seem to stop staring at the Mobilium Prime song until Thalleous breaks them out of their trance.
So if Thalleous had used a Prime song, possibly Aggressium Prime, he could have been influenced more deeply, leading to the sleeplessness and other unstated effects.