Well, with the release of Demon Hunters and the Broken Shore event on tuesday, we also got the release of demonic invasions of six familiar zones in Kalimdor and the Eastern Kingdoms.
While most people will probably be doing these invasions at level 100, you can actually start them as early as level 10. Completing invasions at a lower level will give you a nice chunk of XP, so if you have any alts who are stuck in some level range that you're not really enjoying (or you just want to get them leveled up quickly and you've either used your 100 boost or they're close enough that you don't want to waste it) you can take them to these zones while they're under invasion.
Invasions persist for four hours in two zones on the map (I believe that as the weeks progress we'll see them in more zones, until the week before the launch, when all six zones will be under constant invasion.)
Upon entering a zone that is under invasion, you'll be put in a scenario that (hopefully) is near the beginning of it, in Stage 1. So don't worry about others completing the invasion before you get there - as long as you show up within the 4-hour window, you'll get to do it (and once that window ends, two other zones will be invaded.)
Invasion zones are Westfall, Dun Morogh, Hillsbrad Foothills, Azshara, Northern Barrens, and Tanaris.
Within the scenario, it doesn't matter if you're Alliance or Horde - your Tauren won't be slaughtered for trespassing in Kharanos and your Draenei won't be beaten to death by the guards in Crossroads. They're happy to get any help they can find. However, once the invasion is finished, you should try to get out of there relatively soon, as you'll soon exit the scenario and find that the other faction isn't all that grateful after all.
There are four stages to each invasion. Stage one doesn't reward anything - you simply kill demons as they show up in town (for Azshara the "town" is just the area outside the northern gates of Orgrimmar.)
Stage two has two or three big demonic lieutenants show up (you'll also get a few of the lesser demons popping up - they'll keep coming until the final boss shows up in stage four.) After they are killed, a commander arrives, acting as a kind of mini boss. Killing this one ends the stage and also rewards a lesser Legion cache.
The Lesser Legion Cache is guaranteed to come with 5 Nethershards - a currency that you can spend at Illidari vendors in each of these zones or in Stormwind/Orgrimmar if you've done the Broken Shore/Demon Hunter starting experience on that character, as well as one piece of armor of your armor type. At level 100, this armor is item level 700, so for most alts, this will be a pretty serious upgrade.
Caches have a relatively small chance to also give you a spec-appropriate item level 700 weapon or a Coalesced Fel. This latter item can be applied to one of these invasion weapons, raising its item level by 5 points, to a maximum of 725.
Stage three is the big one - Fel structures and demons will spawn all over the zone, and you'll have to go defeat them. Demons come in myriad varieties. There are minor, non-elites that can be found all over the place. There are also some elites that you'll want to be pretty well geared to take on solo. There are also full-on bosses, indicated by skulls on the map. Killing bosses will give you a few Nethershards - I've gotten anywhere from 1 to 15 from a single boss.
There are three kinds of structures that show up as well. Small outposts are typically surrounded by about ten demons and have a single Fel crystal inside that you can destroy to cause the building to explode (you'll be thrown clear with no damage from the explosion.)
Barracks are larger versions of the outposts. These have three Fel crystals surrounding them, keeping the door shut. Each is protected by a caster who should be killed before you break the crystal. These crystals will fight back, so you want to take them out quickly. When all three crystals are destroyed, the door will open and you'll be able to fight the barracks' elite boss and two adds. Killing the boss will destroy the barracks, giving you a couple Nethershards and again, throwing you clear.
Portals are the largest structures, and each will have multiple elite demons surrounding them as well as a boss-level demon (not independently indicated on the map, but still dropping Nethershards.) Portals have two powering fel crystals that are protected like the ones for the barracks. Shatter them and the portal will be destroyed.
During stage three you can often find helpful NPCs that you can rescue, often inspiring them to join you. You can also find siege vehicles to help attack the demons.
You'll see new buildings and demons pop up farther out after the first 30% or so of the progress meter in stage three. When the stage ends, everyone gets 10 Nethershards and you should start heading back to the main town (for Dun Morogh, we're counting Kharanos as the main town, as opposed to Ironforge.)
Stage four is a fight against the demons' boss. Like stage two, the actual boss demon is randomized, and some are definitely easier than others. Thankfully, the graveyard nearest the town (or the nearest friendly one - such as Southshore for Alliance players in Hillsbrad, which is where they'll spawn if they die in Tarren Mill) has a battleground-style auto-rez system, so you'll get full health and won't have to deal with the progressive death timer if you die a lot to the bosses.
Slaying the boss of the invasion will end the event and reward you with a Greater Legion Cache. The only real difference between this and the Lesser variety is that you'll get more Nethershards (I think 15.) If you have a greater chance at getting weapons or Colaesced Fel from these, I don't really know, and it doesn't seem to be the case.
With the invasion over, you'll be ready to spend those hard-earned Nethershards. There are Illidari camps in each of the invasion zones, and if you've done the Broken Shore event or you're a Demon Hunter, there will also be camps in Stormwind and Orgrimmar. The captive Wormtongue (a new type of demon) will act as a vendor.
You can buy a number of things here:
Fel Bat Pup pet: 150
Coalesced Fel: 150
Armor set Transmog Appearances: 200 - if you've had bad luck getting some piece of armor, like you keep getting mail shoulders but never the helmet, or you're just running these invasions on one character but still want the looks for future transmog, this will give you the full set of appearances, but not the gear itself.
Cloaks, Rings, Necklaces, and Trinkets: 50 - You'll see two options for rings and necklaces, which now use the Legion-era stat allotment, meaning stamina and two secondary stats, but no primary throughput stats (but lots of the secondary stats to make them worth your while.) The cloak likewise has Strength, Agility, and Intellect, making it similarly universal. There is then a Strength, Agility, and Intellect trinket
Blade of the Fallen: 200 - The only invasion weapon that can be purchased with Nethershards, this is Warglaive that is Bind on Account so that you can give it to your Demon Hunter if you're getting a physical copy of the expansion or for some other reason can't create your Demon Hunter character yet.
The quest chain that follows the aftermath of the Broken Shore will direct you to start fighting off these invasions, but while the event is active you'll be able to participate regardless of whether you've done that quest or not.