So, we made a slightly early start on the Stars Going Dark yesterday. Unfortunately, I have to say that neither myself nor the group were particularly impressed.
The problem seems to be the way the monsters are balanced. The impression we were left with was that they're designed to shut down PC actions efficiently, but not to deal that much damage. Which is probably the designers' idea on how to make them a threat while not creating high-level nuclear war, which is a laudable goal, but it did result in some very tedious fights where PCs were shut down and couldn't act but didn't feel any sense of tension or danger.
I did try doing the math, but I'm not sure about it. At level 17, a Fighter will have at least 170 hit points, probably closer to 220 if they have placed their boosts in anything but the daftest fashion. A Rune Giant's average single attack damage is 33; if we assume it has a 50% chance to hit a PC's AC unmodified, that's 18 average damage per attack if we allow for the possibility of a crit. Thanks to MAPs, multiple attacks in the same round will do less damage. So for a Rune Giant to kill a single martial PC will take it at least 12 hits, but that's if it gets triple attacks every round and the PC just stays there until they are killed, which they probably won't.
The first fight actually had more of a problem with the "weird alternate plane" issue. The Rune Giants have mostly charm abilities but with the battle taking place in the mental world with nothing else around, there was very little that the PCs could do to help the giants. We wondered what would happen if the giants had successfully charmed all the PCs - since the fight cannot end until all the giants are dead, the PCs cannot attack because they are charmed, and the giants will not attack because this would break the charm, presumably the giants just sit down with the PCs to discuss insane philosophy until they all die of starvation.
So the PCs snapped back and forth between being charmed/dominated while the giants generally missed them or did relatively insignificant damage with their swords and the PCs grew more and more bored at effectively losing turns at random. Not a whole lot of fun.
After eventually overcoming the giants, it's on to Mr Not-Cthulhu. This had a rather hilarious prequel in that the PCs, on seeing the Ashen Man, assumed he was Ramlock and didn't let him finish speaking before they attempted to use Intimidate / Scare to Death on him. Since he has no stats (note: just because a creature is "not meant to be fought" doesn't mean they won't be), I adlibbed that they failed, but one player then asked if they could use their Wish to wish that the intimidator had rolled a crit. Since True Strike is a level 1 spell effect I figured that could be reasonable for spending a level 10 spell slot, but the next player then spend their Wish to wish that the Man rolled a 1 on his save, and the Ashen Man was scared to death and exploded in a cloud of dust (note: if you haven't given a creature any stats, you haven't given it any immunities). In the interest of keeping the playtest vaguely on track (and explaining why I was laughing at the time), I pointed out they had just killed an immortal harbinger of chaos from another plane who wasn't supposed to be a combat encounter, and we decided that a) Wish couldn't force a dice roll, b) the players could have their Wishes back, and c) the Ashen Man was still exploded because having him stand around spouting deepisms that would make a teen goth cringe is not a good way of making the encounter impressive.
So, up popped Not-Cthulhu! The players rolled initiative, got a few hits on him, and then he grabbed everyone with his face tentacles. A few PCs broke free... and then the ones who didn't break free were in the tedium game again. Every round, make a few escape attempts, make some regular attacks, or cast spells that would often be countered, then take 18 damage which nobody was really bothered by, while the monster used his remaining two actions on the escaped PCs who were more of a threat. This usually involved either using telepathic contact (which, as in the other thread by Colette Brunel, wasn't really clear - it seemed to imply it could do this as a free action because it's communication, so it essentially got a free action mind warping attempt on each PC once) or the at-will Warp Mind.
(The business about the PCs being rendered "unconscious" by the superior constrict - as mentioned previously, we couldn't deal with it because the confusion over those rules. The PCs fell unconscious, they made recovery saving throws against flat values of 10 plus their dying level which was 0, then woke up again and wondered what the big deal was.)
This resulted in one PC being permanently confused, and another PC (the cleric) being locked down by being hit with Warp Mind every turn (thanks to the partial confusion on non-crit save) to prevent them breaking the other PCs out of their conditions. But again we then reached deadlock over whether or not the act of casting Warp Mind would be interpreted as an attack by the target and trigger the auto-response feature of confusion. Regardless, it didn't actually seem a great idea for the monster to confuse enemies! Since the monster is the nearest creature to them, and they want to attack it, that means that a confused opponent is still attacking it 50% of the time (2 or 4 rolled) and only attacking itself 25% of the time; and if the monster attacks the confused target it will attack back 100% of the time, which in this encounter is likely what a martial target would have been doing anyway. Again, we had the image of the star-spawn having successfully confused everybody but then getting killed by the party anyway because the dice aren't in its favor.
Most of the casters were incredibly frustrated at being repeatedly shut down by Mental Dominance (because Verbal Casting has the Concentrate trait), and in fact, most of them just proceeded to run away in an attempt to get out of the aura and cast Fireballs.
Dialog of the moment was:
Wizard player: "Can I wish this monster didn't exist?"
GM: "I wish this monster didn't exist."
So, eventually it was beaten down and the players generally agreed they didn't want to proceed with the adventure, especially when I admitted to them that it's really just a train of further high level encounters that would likely have the same problem. Oh look, Shoggoth. Oh look, Engulfed, another ability to shut PCs down - at least the damage is a bit better so there'd be more tension.