At long last, I have played Esteren and can report on the group's experience with the first half of Loch Varn!
Quick recap of the PCs:
Ean Harn: An itinerant priest from Gwidre.
Ned: A locksmith from Melwan who has just converted to the Temple.
Saeth: A paranoid occultist from Reizh.
Nesa: A young engineering prodigy from Reizh.
Ninel: A Tarish factory worker from Reizh.
I started the session by narrating the bewildering opening, accompanied by discordant Silent Hill music. While Nesa was scrambling over the debris, she was jumped by Terent (I had come to an agreement with the player beforehand that she had a long lost brother), and as soon as I asked her what she would do, she said she shot him, so that saved me some trouble because I was worried I would have to cajole the party into killing this guy.

My trouble in that regard was just beginning, though.
I had a little trouble in getting the party through the maze just because I have had almost 0 GM experience before this and decided not to spend a ton of time plotting exactly what I wanted to happen to them in the maze. I had a list of potential complications to throw their way to keep them on their toes, but I was really nervous about screwing things up (like killing the party right off the bat, or just not making things fun). They all failed a Feat check crossing some unstable debris and got injured (I rolled low, so it was only 1 point of damage). Then I had them get ambushed by some fighters that they tried to talk with (as soon as they started talking, I had the fighters charge

). Then I threw some more unstable debris in the way: A pile of debris propping up unstable steel girders above their heads. Nesa, the inventor, used some rope to try to pull down the unstable girders so they wouldn't fall on the party, but she failed the check (Shooting and Throwing modified by Creativity). Since she failed while using a different Way, I narrated her accidentally pulling the girder down onto herself.
I actually crippled the maze fighters pretty heavily by having them already be heavily wounded (only 5 hp), but the very first round of combat had one of them roll a 10 on the attack on the priest, doing 15 damage... ouch! In the end, though, the priest turned them into ice.
I'd been having them run through different intersections (that I just made up on the fly), and when I felt like they'd had enough I had them discover their campsite at the next turn. I think I did a pretty good job at making them feel constantly unsafe throughout the maze.
The campsite scene was fun, but difficult since my players weren't having any of what Jearon was telling them. The priest, Ean, initially refused to talk until Jearon told him his name. However, I didn't give them much of a chance to interrogate Jearon or Deorn, because I tried to keep the party on the defensive by bombarding them with various things, like Deorn's warning, the warrior starting to panic, bringing up the map, etc. So in the end, I think the scene turned out really well, and I didn't have to let Jearon or Deorn give their names, so I consider it a win.
They ran back through the maze without incident and reached the refinery. I absolutely loved this part. They did some exploration, finding the two documents, and then the occultist, Saeth, got grabbed by the carnivorous plant. The others tried to grab him, but couldn't, and he quickly got pulled into the vat while the others fought off the creepers. Nesa, who had been at the top of an adjacent vat exploring, quickly threw together a molotov cocktail using lamp oil and threw it into the vat, which required a lot of improvisation on my part. You can see my ruling at the bottom of the report. I thought it was a great use of resources. As I suspected, the priest went up into the vat to rescue Saeth. Unfortunately, even though the plant was taking ongoing damage from the flames, Ean was unable to defeat the plant before it dissolved Saeth's flesh from his body. Everyone made a mental resistance check to avoid being scarred by Saeth's horrible screams. I subtly prodded the group to run away, but Ean wanted to try to save Saeth, thinking he could still be alive, but I ruled that, as he hacked away at the plant's mouth, he could see through its tattered leaves Saeth's horribly mutilated corpse, provoking yet another mental resistance check.
They went into the back of the facility, where the exit was supposed to be. However, the only found a supply closet and a restroom... Saeth had had the map on him, so they couldn't check it again! As they searched for another exit, they were ambushed by the giant centipede. At the same time, an exterior wall collapsed and hundreds of bugs swarmed in. The giant centipede killed Ned in one hit, slicing his body in half with its mandibles, then killed Ean as Nesa and Ninel ran towards the exit. As they tried to escape, though, they were swarmed with bugs and killed.
Act II:
When I had the PCs all wake up to Nesa screaming, a mighty groan arose from the table as everyone's idea that this had all been a dream sequence was confirmed.

We reintroduced the characters, and I told them all the basic information that they all remembered.
The best parts of this act have been when the players all had different information. I printed out slips of paper for each player to read, and all I had to do was sit back and watch everyone accuse one another. The paranoid occultist remembered that Jearon was missing and accused the priest of having done something with him, for example, while Ninel believed that Jearon was killed by the monsters she believed they fought in the factory.
The flashback scenes were a fun diversion, though there was a little confusion about what was happening and when. The group's suspicions about the priest weren't helped by the fact that Ninel remembered that they had found Ean at the chapel with two other clerics (Jaber the Vector and Urvan the Blade Knight), which Ean vehemently denied.
Fortunately, the players decided to go to the chapel, remembering the clues they had picked up about some secret there. I was worried that they would miss out on this great scene and the clues there, but with a couple reminders from me and a little prodding (like telling them that they were camped closer to the chapel than to Varn), the PCs who wanted to investigate the chapel convinced the more skeptical ones to check it out. While the Temple faithful were praying, two of the Reizhites discovered the trap door underneath the flagstone. After reading the Temple registers, Nesa came up with the wild hypothesis that the local Demorthen had used the experimental Flux from the Magientist laboratory to create monstrous plants and animals (creatures of nature) to fight back against the Magientists. I certainly wasn't expecting anyone to come up with that.
I was very worried that the party was going to head back into the factory, since a few of them were lobbying heavily for that so that they could investigate what really happened. Fortunately, after I reminded them that they also came here to investigate Varn village, they more or less came to a consensus to head there first.
Unfortunately, we had to stop there because we ran out of time, but I thought that was a good place to stop. Unfortunately, the two flashback scenes took much longer than I expected since the players couldn't agree on anything, but I think we can finish the adventure next time.
Oh, and I just remembered that two of my players had bought guard dogs, but I always forgot about them and thus never included them on anything.
Good things:
-The players were wonderful in their roles, especially during the scenes where they all had different information. I absolutely loved seeing what sorts of ideas and hypotheses they all came up with to account for their lack of consistent information.
-I feel like I handled the players' improvisation well. Nesa's player came up with some pretty inventive things, and I want the players to be creative and improvise, so I rolled with everything. The system works pretty well with this because of 1) how general the Domains are and 2) being able to allow players to use different Ways depending on the situation.
-I found that I had to fudge die rolls a lot, which I don't like to do, to keep the PCs from dying at the beginning of the scenario, but I found that it helped a lot. Besides, combat is so brutal that I doubt that players had any idea that I was taking it easy on them!
Concerns:
-Running this scenario is WAY harder than I expected. After the first PC died, I was worried that the player was going to storm out angrily, so it was hard to not just blurt out, "It's okay! You're not really dead!"
Ruling on the molotov cocktail:
I told the player to roll to see how quickly she could make the item, since we're all still used to being able to move and attack in a round. With her roll, she successfully made the item but it took the whole round. On the next round, she threw it into the adjacent vat. Since the target was immobile, and the weapon was an area of effect, I ruled it the following way: Make a Shooting and Throwing + Combativeness roll vs. a target DC to see if it lands where she wants. Since the target was so close (in an adjacent vat), and she didn't need to arc it much, I set the DT at Standard (11). She rolled a 12, so the molotov landed in the center of the vat and exploded, setting the plant aflame. For damage, I told her to roll a d10, making it equivalent to a first or second level magic power. I ruled that since the target was a plant, it was especially susceptible to fire and didn't get a resistance roll.