Archon (Are-Con)

Most daemons, whilst dangerous, are only a major threat in swarms. Archons are the exception. They are one of the few entities assigned the Existential threat level by the Hunting & Ecological Balance Commission, advising evacuation of the immediate area & divine intervention requested immediately.
Why are Archons such a large threat?
Simply put, magic. Where the rank & files of daemons have abilities that seem magical at a glance, experts who have studied the anatomical workings of them have deduced that each case is a matter of carefully designed intent. As such, whilst their abilities are terrifying, they aren't capable of adapting beyond that which they already have. Whilst archons haven't been studied in as much depth, the few feats observed by them far exceeds the knowledge of even Arcanus, the very God who has made it their mission to chronicle all known magical artforms. As such, the only assumption on the table is that they have some hitherto unknown form of magic at their disposal, allowing them to adapt where their lesser kin cannot.
On top of this, the archons are reportedly capable of creating new daemons out of the living. Even if there are limitations on the who, how & how long, should this report be accurate, the danger they are to their immediate area is only exacerbated as their victims become further perpetrators.
When have they been sighted?
To date, none have been known to have entered Eltar, and all the data on these archons come from other planets where full-blown daemonic incursions have a higher likelihood of success. The most notable cases are the fall of Leris and the failed incursion on Karrix, with both cases being attributed to the first known archon, Sylannei. With the information gathered from those incidents, it's also believed that an archon may be present on Terra, however this has yet to be confirmed.
Regardless of how little we know, it's important not to understate the threat they present, and how likely it is that there are other archons at work elsewhere, either waiting to make their moves or already having done so with us having no intel on the situation.

How do we defend ourselves from them?
From our records, it appears that archons require a significantly greater amount of aether to sustain themselves, and this works to hinder their activities within the material plane. Sylannei wasn't sighted until the latter portions of the fall of Leris, & her presence during the Karrix invasion was through a proxy she'd linked her mind with. Charged anterria crystals proved effective at destroying lesser daemons & appeared debilitating to the proxy, however given Leris was the main source of anterria crystals this option isn't sustainable, and suggests Leris was specifically targeted to gain control of this resource. Her defeat on Karrix was a result of the aethergate being disabled, and depending on the model of aethergate is a permanent solution to all forms of incursion by daemons. With these being the only two weaknesses known, until we have a more concrete understanding of the extent of an archon's power, we have no clear understanding of how to best fight them. 
Given the unknown factor at play, Shieldguard numbers are being increased, with at least two archmages keeping watch over the rift at all times, which should provide a strong enough defence to get a word of warning out and divine intervention requested.