Im super weirded out by the encounter however.
The Yochol makes two attacks per round with 10' reach (at +6) dealing 7d6+2 damage a hit. It's resistant to non magic weapons (and a ton of other stuff) effectively giving it 260 hit points. It has advantage on saving throws vs spells. It casts dominate person (DC14) 1/day.
How on earth was it NOT killing 1 PC per turn?
Even if it gets surrounded (it can spider climb) it can cast Web (restraining the party) at will - which it is immune to, and then turn into gas form (where it is invulnerable to non magic attacks) and simply.. float away.
The two drow elite warriors have ACs of 21 with parry (assuming you didnt give them drow armor), 70 HP each, two attacks per round doing 4d6+4 damage. Thats after the volley of knockout dart crossbow bots (in darkness so probably at advantage) against the PCs.
The priestess is spamming a 20' radius (40' diameter) sphere dealing 4d10 damage per round (con save for half) while she melees for 2 attacks per round at +5 dealing 6d6 damage per hit. If they drop her concentration she can simply put it up again.
Just those 4 critters alone should have wiped the floor with 8 x 5th level PC's - they should have been dropping like flies!
We're talking about (just the 2 elites, the yochol and priestess) throwing down 8 attacks per round dealing an average of 6d6 damage per hit each, plus an AoE spell dealing 4d10 to a bunch of people.
Assuming just half those attacks hit, thats 2 PCs going down per round (before you factor in the insect swarm).
The preistess has mass cure wounds too.
Yeah, it sure looks impressive.
But you must not have my players - much like the D&D designers, I suspect!
First off: the summoning kerfuffle. The adventure specifies the Priestess tries to summon the demon only when "bloodied" (I'm using this very convenient 4E term for "at half hp"). And then, as has been correctly pointed out, any summons poofs out when the conjuror is downed.
Rather than doing this by the book for a very underwhelming scenario, I had Ilvara use the one round the Drow got when the party fireballed the initial guard post to call the Yochlol. Then I had the demon just sit there and be amused at all the hurting the party inflicted on the Drow. The she-demon would only enter combat when attacked or when Ilvara got bloodied and (as a free action) cried out for help. The Yochlol would then fight to the death. It would not disappear when the Priestess died, which I believe in previous-edition parlance would be called "calling" rather than "summoning" the demon. Anyway, the important bit is that I consider that to firmly justify including the Yochlol's full CR 10 to the encounter difficulty.
The fight went down as follows:
Ilvara casts Insect Plague to plug the passageway into the cave the Drow is using as camp. All characters eventually take damage from this, but notably only once (much like Stinking Cloud, the 5E version of this spell is in my opinion not worth its level for this reason).
The party fireballs the camp (unknowingly frying the gnome they're trying to rescue
) clearing out the non-elite drow.
Then general melee ensues. The monk and the PC fighter engages Ilvara. The two Drow elites hope to attack the "soft underbelly" of the party, and Shoor does manage to down the NPC fighter in the party. To me it's obvious he would not waste a precious round on killing off a fallen foe when he's still in melee with active enemies, but each to his own. The elite ability to go from AC 18 to 21 on one attack per round did cause quite the consternation, but the party had quite a few tricks of their own. Once they set up attacking him with advantage, they reliably get in quite a few attacks each round. The battlemaster ability that says "attack me or attack at disadvantage" is very effective in neutralizing a specific foe (since it's used by an archer more than 30 feet away).
Jorlan is quickly bloodied when all the archers focus-fire on him when he seriously threatens the party Warlock, and at this stage he calmly states "I'm going to go away now, anyone standing in my way will be killed". They let him run off (but caught and killed him later). I should add that I did not apply the handicaps of the adventure - I did describe him as burned and in pain, but did not want him to be effectively useless (by applying disadvantage to his attacks). I did express his disloyalty by caring only for himself (which would have worked against any other party than one with a super-spy familiar and a 50 ft Speed monk as backup).
Back to the fight: Ilvara held her own and bloodied one of her attackers, but then quickly went down. The monk's stunlock is a very effective win button. Besides; she isn't equipped with many scary spells once she's cast Insect Plague. I don't think it changes the CR calculation, but I did not give her the Tentacle Staff.
The Yochlol did try to keep away from being surounded. I had prepared the cave to allow it to reach them from the ceiling. I did allow them to cast Shatter at her square, though. Ruling that she could lose her grip when chunks of ceiling fell off, she failed an improvised Dex save and fell down in their midst. And that was that.
As to your specific concerns:
The monk used bonus dodge to save his life (making the Yochlol miss half the time). It still reduced him to single digit hit points and forced him out of the fight. But one round of stun and one round of lying prone (from falling to the floor) means an awful lot of attacks with disadvantage. Obviously the PCs without magical attack means spent their attacks somewhere else. I don't think I've handed out a lot more magical weaponry than the adventure allows. They have the gelatinous cube mace, one dagger and one short sword, I believe. All magical drowcraft items. I've given them six +1 bolts, but that could be on me (not sure). Eldritch Blast does full damage (with no save).
In the end, a lot of hit points won't save you if you're all alone. Remember, when the Yochlol "activated", Ilvara had already gone from bloodied to dead before the demon could act. (The party reliably dishes out a hundred damage per round -- doesn't yours?)
But back to your issues: I remember trying to Dominate the kill machine of the party. Had she not rolled well, that could have meant a lot more pain for the party. But since the spell failed, that was one whole round wasted on not dishing out those sweet sweet 14d6 damage... (I suspect a Yochlol should never waste any time on that spell unless it can cast it at range, well before its foes can enter melee. Unless it fights against level-appropriate opposition, of course! Dominating a level 12 hero, say, is a much more worthwhile endeavour than the (by that time small) prospect of dishing out a (by that time piddling) 14d6 damage in tier iii play).
I will grant you that if the Yochlol would have wanted to escape, it would have had an excellent chance of doing so. But a Yochlol there only as a "combat summon" doesn't think of retreat if you ask me. After all, it isn't risking real death.
The party cast Daylight. Not only does this mean any Darkness evaporates instantly, it means Drow Elites attack at disadvantage. (I'm not actually sure how much of "real daylight" this spell confers, but I didn't want to rule their high-level slot worthless, so...)
Not sure what you mean by "spamming a 4d10 sphere". Are you talking about an aura spell?
Because if you're talking about Insect Plague, she does not cast it centred on herself. And she can't cast it and attack in the same round. And as soon as she's in melee, anything with "concentration" is worthless, since she'll lose the spells faster than she can cast them. That strategy would turn her into a sitting duck. Even if she could cast Insect Plague every round, and even if I thought it be fun for her to optimize her behaviour on this spell, she would have had to suffer opportunity attacks to move out of her circle of attackers, drop Insect plague, be attacked and lose Concentration, move out again... I grant you she could concievably deal more Insecty damage that way, but she would sure go down faster with all those OAs.
Finally, you don't even mention Asha, and you'd be right to do so. The only action available to the NPC Priest creature that would make any difference at all (as far as I can see) is healing a more capable ally, thus forcing the heroes to waste half a round of attacks on killing her. In the end, I interpreted the adventure as to mean she didn't want to heal Ilvara, so she kind of tried to sneak away as soon as she saw her side was losing. I can't remember what did her in, when she failed to climb out of the cave's back entrance.
In the end, the players were suitably impressed. They called it a "end level fight".
As for me, I conclude what the DMG calls "double deadly" I call "very hard, bordering on deadly"
