The first wing of the Emerald Nightmare LFR is up, and I have now experienced the first three bosses.
While these are the first three on LFR, it seems apparent that on Normal or above, you will be able to do bosses 2-5 in any order. I believe that Nythendra, Cenarius, and Xavius are all locked at positions 1, 6, and 7 in all modes, but the other bosses should be available to do in any order on non-LFR runs.
The other note is that I was unable to loot the quest item for the "In Nightmares" quest. While I had suspected that this might have a Normal-and-above difficulty requirement, I'm not certain, as the quest makes no indication of this and also the various bosses did have quest objectives in their tooltip. I'm waiting for a clarification from Blizzard.
So let's get started!
The first boss can be fought very soon - there are a few bits of trash before her, namely globs of corruption that will do some knockbacks, aoe, and spawn dangerous insects that pulse damage. We had a rogue pull nearly all of them, which resulted in a near wipe (way to start off LFR well this expansion, player.)
Nythendra is actually fairly simple. She will periodically put "Rot" on random players, who, after a short duration, will spawn a pool of rot where they're standing. You want to keep these clear of the melee, but also remember where you put them.
Tanks get a super-charged version of this called Volatile Rot. The current tank will get this debuff, and they'll need to run away while the other tank taunts. They will then spawn a pool of grossness, but if they're within something like 30 yards of anyone else... something bad will happen (it might spawn rot under them as well.) Basically, treat it like artillery from the Iron Reaver in HFC.
She'll also do a breath attack, but she remains pointed in the same direction while she does it. So tanks should face her away from the raid and then dodge the breath when it goes off.
I think simply based on time Nythendra will go into phase two. She'll stop attacking and instead start to draw the pools of rot back to her - meaning that everyone will need to dodge the rot as it comes back toward them.
During this phase she'll also spawn large bugs that will radiate heavy damage, so you'll need to dodge the moving pools and the stationary bugs (the bugs are really more of a spell effect - they can't attack or be attacked.)
Once all the rots are reabsorbed, she'll revert to phase 1.
Il'gynoth, Heart of Corruption:
In the Nightmare version of Un'goro Crater, Il'gynoth is the closest we've gotten to an Old God fight since Yogg-Saron.
As a tank this fight is kind of strange, as there are periods where there's nothing to tank.
At the pull, there will be a big eye in the hollow of the tree which holds the heart. This can't be killed conventionally. There will also be two Destructor Tentacles that need to be tanked.
Over the course of the fight, players will have oozes fixate on them. In LFR at least, you simply drag these oozes to the eye and kill them there, as when they die, they'll damage the eye.
You will also get other tentacles that have eyes and will Mind Flay random players, but this can be interrupted and should be.
Additionally, large mist-giant adds will spawn as well. These need to be tanked and have a debuff they put on the tank that demands a tank-swap.
Once the eye is killed using the oozes, everyone should pile in (maybe kill any adds that are up) to the tree and start attacking the heart. During this phase, random players will be given a debuff that causes them to damage players around them after a few seconds, so simply run to the edge of the room until the debuff expires.
The Heart will slowly cast "Nightmare Reconstitution" during this phase, and if you ever fought Yogg-Saron, this works the same way as the thing the brain is casting after you have entered it. Once the cast is complete, everyone still inside the tree will be insta-killed and the eye will come back. So watch that cast bar and when there's about ten seconds left (about when the bar passes the "n" at the end on "Reconstitution") you want to get the hell out of there.
Then things revert to phase 1. I don't remember if it happens more frequently than this, but certainly after each heart phase, you'll get two new Destructor Tentacles (not in their previous locations) that should be tanked.
Bad stuff on the ground seems to be a theme in this raid.
Elerethe goes through two phases. She begins in spider form. During this phase she'll poison random players and cause them to drop poison on the ground. She'll also periodically summon spiderlings as well as go up into her web and then drop down, damaging people within the circle that shows where she's going to land.
When the spiderlings spawn, have one tank pick them up while the other holds onto the boss. The spiderlings will debuff the add-tank, meaning that the next time they spawn, you'll want the previous add-tank to pick up the boss while the other tank goes and picks up adds.
After a certain amount of time, Elerethe will switch to Roc form.
She will do a nasty attack called Raking Talons that debuffs the tanks, requiring a taunt-swap (which should happen immediately after she does it.)
She will also summon purple tornados that stun you and do massive damage. After a bit, she will take to the skies, pushing players back but also leaving behind wings. These will let you jump very high and far. She will fly to one of the other bluffs, usually leaving tornadoes on one of the web-bridges between them, meaning you'll have to navigate around them to get to her.
With Elerethe down, you'll be done with wing one of the instance.
In terms of difficulty tuning, it does not seem too bad. We had one wipe on Il'gynoth and one on Elerethe, but this was literally the first time that most of our raid had seen these fights (myself included.) As familiarity (and gear) grows, I doubt these will be terribly difficult.