Why is lyrium required for the Harrowing?

I'm guessing the lyrium is used for mages to enter a trance, where they do the harrowing ritual. So, sort of like dreaming, but awake in an altered state of consciousness. Dreamer mages don't really need lyrium, they can enter the fade at will, or they are maybe always connected to it while awake, but most mages aren't dreamers, so that's not an option for everyone. The senior mages and Templars are there to guide the journey and monitor in case of demon possession, so they are observing the ritual and watching for signs.

Why use such an expensive mineral just so they don't have to wait for the mage to fall asleep, because it's an important rite of passage and an initiation ceremony. Plus there are high stakes if it doesn't go right, considering failure can mean death for the mage.

Based on what we see from Wynne and a mage Inquisitor, it seems that, although mages remain fully aware in the Fade and remember it as if they were awake, they can't necessarily tell when they're in the Fade. The lyrium likely holds the role of making sure that the apprentice falls asleep on command, and making sure that they know they're in the Fade.

As others have said, I'm guessing it's to help with the sort of lucid dreaming journey they take during the Harrowing?

Personally, I'm wondering what keeps mages from becoming lyrium addicts themselves.

Oh, I didn;t think about the Circle mage getting sick with it, and how that would look. I bet it would look like possession, or be assumed to be that.

And then if lyrium is such an integral part of a mages toolkit, why is it presented during the Harrowing like it's a big deal? It must be something else to the ritual we're not seeing because mages take lyrium all the time, yet they don't automatically travel the Fade when they do.

So, there's something else going on there.

edit: oh, and maybe whatever it is that makes mages, mages, also provides them with some natural resistance, at least, to lyrium addiction?

I think the Codex in Origins established that mages can get addicted to lyrium. As for why they mostly don't, Templars seem to take it more often. I think I remember meeting a carta member or something in Haven, and that person saying that mages and Templars have very different buying patterns with regards to lyrium; Templars use less when they use but use a lot more often, whereas mages can go without it indefinitely until they suddenly need a lot more power than they naturally have and buy a whole lot more lyrium than any Templar would have to at one time.

My guess is that a mage gets addicted if they frequently need more power than they naturally have. (As I recall the Codex's example of addicted magi was some Tevinter Magisters, which seems to fit that description pretty well.)

Though this doesn't necessarily preclude mages being more resistant due to being innately magical.

On topic: The ritual with the lyrium seems to allow a mage to go to one specific area of the Fade, since that's how and why the ritual is used in The Arl of Redcliffe. I've been wondering for a long time if the Circle is arranging for the mage to run into demons during the Harrowing, since otherwise the test could be rendered moot by a mage running into nothing but benevolent (or weak or timid) spirits. I've been trying to figure out how the Circle could arrange for the demons to be where the mage ends up, but maybe it's the reverse: the Circle looks for someplace that suits their purposes and then sends the mage there. And exactly where in the Fade they end up something that a mage who just falls asleep could credibly be unable to control that well... unless that mage is a somniari, which is supposed to be an extremely rare gift. (Not to mention that the mage ending up wherever they want isn't quite the point: in this interpretation of the ritual, the Circle and Templars want to control where that mage ends up.)

^^ It seems to be that the Harrowing is supposed to be a mage's first real trip to the fade. It's their initiation into becoming a mage. Remember they are usually taken to the Circle at a young age, so this is likely their first ritual into the fade itself. Outside of their natural encounters while dreaming, of course. Given that Mouse claims to have been a former mage who failed his Harrowing, it's possible the Circle actually has a deal with this demon to perform initiations on their mages, so it's Mouse who is setting it all up. That seems to be what Valor implies, when he questions why mages pit their apprentices against demons. So they definitely expect demons to be encountered. Or it could be that a mage's first trip into the fade just naturally attracts attention from the fade spirits and demons. They are crossing the Veil for the first time and becoming visible to them.

So it seems the general consensus is that under normal circumstances, mages enter the fade via dreams but are not necessarily aware that they are in the fade. They do however have the ability to remember their fade visits upon waking. What the liquid lyrium does is that it allows the mage to enter the fade while fully cognizant.

At first glance it seems rather counterintuitive. If the harrowing is meant to test if they can resist possession, then allowing them full consciousness when they are usually devoid of it does not seem to actually test their ability. Or at least not as reliably. It's like testing the ability of how well someone's eyesight is only to equip them with glasses. Or testing how well someone swims only to equip them with a swimsuit.

So I assume there is something else to it. Something that actually increases their risk of possession--or at least the probability of a powerful demon encounter. The most common guess is that the ritual also involves the deliberate summoning/attraction of a demon. But....maybe their consciousness in the fade is a danger in and of itself?
If dreamers are any indication, it is possible their now-heightened presence in the fade attracts demons more than usual. The presence of demons is also known to aversely affect dreamers for some unknown reason which implies demons can affect them more than normal dreaming mages.

So maybe just like some posters have guessed, full consciousness in the fade also has the affect of allowing demons to more easily "see" the harrowing mage in question. In which case, it can be theorized that the demons are not only able to find the mage more easily, but they may also be able to gather more 'ammunition' (better insight into the mage's life/personality) which they can then use in their possession attempt.

That much I was able to glean, but there were times when I confused due to the contradictory nature of other dreaming sequences--in no small part due to how many mages are "special" in some manner. I was quite mired in what constituted "normal" dreaming encounters.

As an example, the player character is typically treated different than other mage characters by having access to player agency (which usually translates as "strong will" in the game). So I try looking toward companions or codex entries.
In DAI we have Solas's approval sequence in the fade, but between the mark and Solas position as a dreamer/evanuris, I try not to view it as a typical dream. Especially since they do not even bother to acknowledge how a dwarven Inquisitor's ability to experience it too.

In the DAO quest Lost In Dreams, a mage like Morrigan managed to identify the Flemeth she saw as a demon, but I convinced myself to view it as an exception due to her special tutelage under Flemeth. Wynne (harboring a spirit of faith) on the other hand was all but mired in a nightmare where a group of apprentices have died. In DA2 a great deal of companions, even mages like Merrill, succumbed to temptation. But then I had to consider they were involved in a Dalish ritual that may have had an effect on how they dreamed. And on the harrowing itself I only managed to find two sources that were not reliant of player experiences. A codex entry found in Here Lies the Abyss and a bit of banter between Cole and Vivienne.

So I was quite flummoxed when I found myself wondering what exactly the 'normal' state of dreaming was in Thedas was.

I think a dwarf Inquisition can experience the Fade with Solas through a combination of Solas' dreamer abilities and the power of the Anchor. The same holds true of a non-mage Inquisitor who equally should not be able to experience the Fade so clearly or interact with Solas as they do.

In DA2 everything was weird about that Fade sequence. Marethari says that to kill Feynriel in the Fade will render him tranquil, so presumably that only applies to Dreamers because we can kill Merrill in the Fade and she suffers no ill effects at all. It is also strange that Marethari seems to have the power to send the whole party into the Fade, when in DAO the combined might of Irving and his fellow mages was only sufficient to send one consciousness through and that person had to be a mage. No mention is made of what Marethari uses to achieve this. Did we supply her with lyrium? Did she use blood magic?

The presence of the faith spirit does not render Wynne immune to the perils of the Fade as she suffers a similar loss of will in Asunder when she appears to lose a battle of wills with the demon possessing Pharamond. Her companions intervene but to no avail, then the faith spirit takes over. So it would seem it only does so when all other options have failed. Therefore, perhaps we should assume that if the Warden had failed in their intervention, the spirit would have stepped in.

The thing I find strangest about the lore on magic and dreaming relates to saarebas. They must be able to access the Fade or they would not be able to do magic. Yet the Qunari teach that entering the Fade is reprehensible, even in sleep. Qunari claim they do not dream as others do and may be it has been possible for them to condition people in such a way that they do not remember it. Yet people also say they rarely encounter Qunari in the Fade which seems to lend credence to their claim. What then becomes of the saarebas? If they are conditioned not to enter the Fade, then they are effectively tranquil, but they quite clearly are not, so they must enter the Fade. When questioned about this, do they lie to their arvaarad? Or is the reason they have their mouths stitched up to remind them they are not meant to talk of their experience in the Fade, even if they have had one, so their leadership can maintain the fiction that no true Qunari enters the Fade?

According to World of Thedas, during the Harrowing the apprentice is surrounded not just by Templars but senior spellcasters as well. No doubt the latter are there to ensure that a demon actually does occupy the area of the Fade that the apprentice enters. I think the whole point of the test is to discover if the mage can see through the illusions that the demon will create in order to win their confidence and thus overcome them. The demon will start off by being friendly to put them off their guard. If the mage proves too clever for these tricks, then the demon will turn hostile and attempt a violent possession. If the apprentice successfully resists all of this, they have proved themselves of strong enough mind not to be a danger in the normal course of events, whether that be casting spells or simply sleeping.