As is the tradition begun with Warlords, the final wing of the LFR raids is the final boss on its own. I wonder if they'll go that route with Trial of Valor, as that would leave only two bosses for the other wing, but we'll find out in... well, three weeks (actually maybe more given that LFR usually comes out after Normal mode.)
The fight here is against Xavius, and as often happens on LFR, particularly when a new wing comes out and everyone brings their mains, who vastly overpower the fight, some of the mechanics got lost in the churn.
But let's talk trash first:
The Rift of Aln is featureless. It's a grey void with a haze of fog around where you're standing.
Within the fog are several minor-Sha-like Nightmare monsters. Attacking them actually sends them away, but then you start to get waves of three elite monsters (either "medium Sha" like monsters or faceless ones) which you'll need to kill.
Eventually, Xavius steps out of the fog, but unlike his rather conventional Satyr look we saw in Val'Sharah (which was really more of a quasi-independent avatar that we destroyed in Darkheart Thicket,) the true Xavius has grown misshapen and monstrous.
There are two major interacting mechanics in the Xavius fight. The first is Corruption - many of Xavius' abilities will cause Corruption to build on players hit by them. You have a little meter at the bottom of the screen to track this.
Hitting various levels of Corruption has various effects (this should all be familiar if you remember the Cho'gall or Sha of Pride fights.) Upon hitting 100%, you'll get a big damage buff for I want to say 30 seconds, but afterward you'll become mind-controlled for the rest of the fight.
However, there's another mechanic at play, which is Dreamstate. At the start of the fight and I think every three minutes, the Dream of Ysera will put roughly half the raid into a Dreamstate. Their bodies will drop to the ground and fall asleep while they will be able to go about as a sort of dream avatar that can do all the things they'd normally do.
If you die in the dreamstate, you simply wake up, and any corruption you suffered during the dreamstate is wiped clear. I don't know what happens if you get to 100% corruption in the dreamstate, but I assume that you simply wake up. It also wears off after 3 minutes.
The trick to the fight is making sure that players who are dreaming are the ones dealing with the corruption. Tanks who are dreaming will pick up adds that cause corruption to their targets, dreaming healers will dispel debuffs that cause corruption to the dispeller, and dps will kill other adds that cause corruption upon death.
Either we pulled off this fight better than I could have hoped for or I was doing something wrong, because I don't think I ever got above 25% corruption.
Upon beating Xavius, you're actually transported to the heart of the dream, which has now been purified (mostly.) You can actually capture a special battle pet in here, which is a Dream Whelpling. You'll also see the Dreams of various bosses you killed, who are now released from the madness. Following Ysera, however, you will find a cave where there is a strange flower that has a void portal above it, called Remnant of the Void. I think the druids are going to have to keep a very close watch on that thing.