A proper Archdemon hasn't been seen since the playable Warden (or one of their Warden comrades) slew Urthemiel in Dragon Age: Origins, with Urthemiel potentially being destroyed alongside a Warden or cast into Kieran, Morrigan's child. However, with the Grey Wardens at an all-time low in Dragon Age: Inquisition, a returning Archdemon would be a major threat, and that's just one of many convenient reasons players may see not one, but two Archdemons in Dragon Age: Dreadwolf.
The remaining Archdemons are known as Razikale and Lusacan, the God of Mystery and Night respectively. Archdemons are unleashed and a Blight begins whenever the Darkspawn find one of the Old Gods somewhere in the Deep Roads, but what exactly the Old Gods are prior to a Darkspawn's corruption has never been revealed. After all, Corypheus and the other Tevinter Magisters breached the Veil and traveled into the Fade at the heed of the Old Gods...which are supposedly just snoozing dragons beneath the surface. There's a lot of lore that can be unpacked here, but chances are, Solas' plan in Dragon Age: Dreadwolf to tear down the Veil involves the Darkspawn to some degree.
Dragon Age: Dreadwolf's Darkspawn Are Inherently Connected to the Veil...
Solas is the one who created the Veil, separating the world of magic from the mortal plane and locking away the Evanuris—ancient, immortals, and supposedly vile rulers of the original Elven people. When humans crossed the Veil into the Fade in the flesh, the Darkspawn were created. Many blame Tevinter for the Darkspawn and the downfall of the Elves, but it's revealed in Dragon Age: Inquisition that the Elves had begun to fall long before Tevinter touched Arlathan. The same can reasonably be applied to the Darkspawn; yes, the Magisters crossed over but if it wasn't the Olds Gods, then the question becomes who was it?
Because of parallels between the Old Gods and the Evanuris (the number of them when discounting Solas and Mythal, their general themes, their religious ties to the chantry, and their association with the Veil in particular, not the Fade as a whole), it's hard to imagine that the Evanuris were not involved with the creation of the Darkspawn.
They may be trapped beyond the Veil, but they are hardly referenced when in the Fade itself, and the only thing known about their power is that it is incredible. They may be trapped there, but it stands to reason that they could have the power to reach beyond it and influence the world, something Dragon Age: Dreadwolf will certainly deal with if so.
Dragon Age: Dreadwolf's Solas Intends to Break the Veil
Now, BioWare could approach the Veil in a number of ways. It could be that players stop Solas from tearing down the Veil throughout the game, it could be that tearing down the Veil kickstarts Dragon Age: Dreadwolf, or it could all be smaller in some more grand plot. However it is approached, tearing down the Veil means dealing with what's beyond it. That likely means facing the Evanuris for Solas, something he must know is a death sentence. Solas in Dragon Age: Inquisition will express disgust at the Warden's actions against the Darkspawn and Archdemons, though he never explains why.
This is likely to be explored in Dragon Age: Dreadwolf, offering much in the way of lore that could tie this all together. A recent Dragon Age: Dreadwolf leak saw players in the Grey Warden HQ of Weisshaupt while it is under attack by Darkspawn, and one rumored Dragon Age: Dreadwolf companion named Davrin appears to be a legit Grey Warden. It's unlikely BioWare pulls a Blackwall twice. And knowing these aspects of the game exist, as well as Solas' plot in general, means the Darkspawn are already known to have a significant role. Perhaps more so than DA2 and DA: Inquisition.
Because of these things, the connection between the Veil, the Darkspawn, and the Evanuris seems stronger. And if something controlling the Darkspawn and Archdemons exist beyond the Veil, it would likely use its powers to raise the final two Archdemons and seek to dominate Thedas for itself. There are a lot of avenues for the story, but if the Veil is coming down, it affects more than the spirits and demons beyond it, but the spirits and demons as they exist in the living world too.
Dragon Age: Dreadwolf is in development.