After you cross the threshold behind DBS, you enter the Upper Spire of Icecrown Citadel.  As you advance forward, there are several gas traps that you have to move through and not stand in before you get to the open area in the middle.  A rogue can disarm them as they move through, otherwise you're left to walk through or blink past them.  The nice thing about this spot is there's a teleporter once you get through the traps and you never have to do them again... in that raid lockout.  As you stand in the doorway, looking out on the actual spire you see two Val'kyr flying around the center.  They can be single pulled and will steal your soul and duplicate you as a severed essence.  If the val'kyr happens to use this on your healer, then the healer essence will heal the Val'kyr and itself unless you KILL IT.  So you should KILL IT.  Once they sever enough essences, it's more worthwhile to start AOEing.

After you've blasted both of those to oblivion, you head to the left and the plague wing of ICC.  At the doorway, a pair of Blighted Abominations are waiting on you.  These guys do a Scourge Hook ability that picks a random person and pulls them over to the Abom's melee range.  They also have a frontal cleave and an AOE Plague Cloud.  Nothing special with these guys, just blast from range and move out of melee if you get hooked in.  The next pack has a pair of Pustulating Horrors and Plague Scientists.  The horrors have to be focus fired down because if they reach a certain percentage, they star casting Blight Bomb which does pretty nasty AOE damage to everyone when he explodes.  There's also a pack of geists running around inside that you can just AOE down.

Once you're inside the stairway room, you can go up either side to the landing at the top.  Here there's bunch more NPCs running around, including a Decaying Colossus who's standing there drooling waiting for you to get his attention.  This guy does an AOE knockback affect that hits everyone, so back yourself up against a wall, making sure your healers have LOS on you.  After he's dead your tanks will pull the NPCs in the front of the hallway very carefully so that you can take a swing at the mini-bosses Precious and Stinky.  These guys do a decimate effect similar to that of Gluth in Naxx.  They also summon zombies that have to be AOEed down.  Most important thing here is again not to LOS your healers as you'll be taking a massive amount of raid damage.  Stinky also has an AOE damage aura.  Some guilds choose to set him up so that the dog is around a corner and the healers and ranged are LOSed from his damage aura while the tanks and melee stand in front of him so they can all be AOE healed.  There's random drop loot from these guys and Precious drops the highly sought after Precious's Ribbon, a green quality pink sleeveless shirt that gives you a flavor buff, but doesn't actually have any effect of value.  There's a few more pulls to clear the hallways and a pair of traps that summon geist packs from the stairway room.  It's a good idea for a tank to be left behind while the hallways are being cleared so that they can pick up the packs should someone hit a trap.  Otherwise you're on to the two abomination twins.  We'll start with...

Rotface is the gimmick abom of the twins.  This fight takes a lot of coordination, kiting, and movement.  The main tank holds the boss in the middle of the room the entire time.  Your main position is behind the boss within his hitbox.  Most raids group up all their DPS and healers in either two or three groups depending on if it's 10 man or 25.  The reason for this groupage is because Rotface will occasionally turn towards the raid and let loose Slime Spray, a frontal cone ability with a very narrow range, but a depth that goes all the way to the wall.  It's easy to side step out of this and if the DPS and healers stay in groups, it limits where he'll turn and shoot, as he picks a random target for the ability.  The only reason you need to be hit by this is for the weekly quest in this wing.  More on this later.  For the meantime, consider it an ability to avoid.  During the fight, various areas of the room will also be flooded with ooze, which slows running speed and does damage.  If you find yourself standing in something green, move out of it.  Blink is your friend.

The third little trick that Rotface has up his sleeve is Mutated Infection.  This is where the second (and occasionally third) tank(s) comes into play.  This ability applies a DOT to a random person in the raid that is not the main tank.  It does significant damage and also applies a healing debuff so it needs to be cleansed, HOWEVER, once it's cleansed a small ooze appears with an AOE damage aura and the ability to spray puddles of goo at it's target.  Plus the little buggers hit pretty hard.  In order to deal with this the off tank (or tanks) is running around the outside of the room kiting oozes.  It helps for the off tank to have a slow and DKs seem well suited to this job, though any tank could probably do it.  Should you be hit with Mutated Infection, get your ass out of the middle of the raid and run in the opposite direction the kiter is moving so that you meet him on the outside.  If you are the first one with an ooze (or the first one after an ooze explosion), then you may have to run with the kiter till another person gets to the outside ring.  The reason behind this is that little oozes cannot be taunted off a player.  They have a one person aggro table.  The big oozes, however, can be taunted and this is what the kiter is responsible for carting around the room.  Oozes of both types can be snared, so Frost Nova, Slow, and Ice Armor are your friends in this situation, but use them wisely and away from the main raid.  Once the oozes have merged and you have a big ooze, said ooze gets stacks of a buff called Unstable Ooze.  It can gain up to 5 stacks before casting Unstable Ooze Explosion and raining down icky green hurt on a random member of the raid and anyone near them.  Generally this will happen in the middle of the room as the majority of the raid should be there in their little groups behind Rotface.  The tank will be taking a lot of damage here, so it's your job not to take damage.  If you move to the outside of the room, you can DPS from there until the explosion has cleared and then quickly make your way back to your designated spot.

For a while, there was a bug where Mirror Image would mess up the aggro of a little ooze and possibly stay in the middle of the raid, but that was hotfixed.  It's best to blow your cooldowns here with bloodlust, which is generally in the beginning of the raid.  As Rotface takes more and more damage, the time between his abilities decreases and your raid has to move faster in their little dance of insanity to keep from wiping.  The most difficult part of this fight is really the coordination.  Once you get it down, it's not that bad.  Once Rotface is dead the brave master looter puts on his gloves and reaches into the corpse for loot...

Viable 10 Man Loot:  Ether-Soaked Bracers, Gloves of Broken Fingers

Viable 25 Man Loot:  Dislodged Foreign Object, Corpse-Impaling Spike, Death Surgeon's Sleeves

That trinket is one of the best available for mages, especially if you don't already have a Reign of the Dead/Unliving.  After you're finished looting and heading to the far side of the hallway, you may run into some trash or another trap with geists.  But it's not a long trip or a lot to clear before you face...

Rotface's ugly twin brother is the DPS check for ICC.  I've heard it said that the standard is something like 6k DPS per person in this fight.  Don't quote me on that, but you do need to whip out your best DPS on this guy so you can avoid his berserk timer because he will SMASH you.  On top of the need for DPS here, you also have to be able to do a little dance for this guy too.  Blizzard loves their movement intensive fights in ICC, don't they!

So when you start the fight, Professor Farnsworth... I mean Putricide... will flood the room with orange gas that does AOE damage to every one.  There's no avoiding this damage and the healers have to heal through it.  Positioning is wierd for this fight as there has to be a certain number of people at range otherwise Festergut will start tossing his Vile Gas into the melee and screw everybody up.  The person hit with this spell will be disoriented, walking around a small space and vomiting everywhere, with the potential to spread the debuff (and vomit) to other players within 8 yards.  Taking into account that you'll be moving around while you spew, you want to give yourself a 10 yard buffer from anyone else in the raid.  Melee are lucky and don't have to deal with this and if you have enough ranged DPS, healers can stand in melee and not have to worry about it either.

While you're pewpewing your heart out at Festergut, he's inhaling the gas on the floor which decreases the raid damage, but increases his attack speed and damage and helps him build up to his big nuke.  There's enough gas in the room for him to get three stacks of inhale before he farts and nukes everyone and in order to avoid the big one shot, Blizzard gives you the Gas Spore mechanic.  Festergut tosses out 2 spores in 10 man (3 in 25 man) on a random player.  After a few seconds these spores explode putting a DOT on players within 8 yards.  When the DOT falls off, you get an Inoculated debuff that decreases the shadow damage done.  If you're lucky, Festergut will toss one spore into the melee pit and the other out in ranged.  When that happens you have 12 seconds to get your ass underneath the spore in range so you can get the DOT and the debuff.  If you should have the spore, move to the designated spot for your raid; keep in mind some raids don't have designated spots and all the ranged may bunch up on you and also if all the spores are at range, you may be asked to move into melee so the tank and melee DPS (and healers in most cases) are covered by the debuff as well.  The nice thing about mages is that we can actually Ice Block through one Pungent Blight and not have to worry about one round of spores.  If you choose to do this, timing is key.  Pungent Blight does have a cast time and no real damage duration, so if you're watching Festergut's cast bar, then you get a free ride.

A few things to keep in mind on this fight:  DPS is key as there's no surviving the enrage after 5 mins.  Tanks are also responsible for a tank swap because of a debuff that will make them explode and kill everyone if they don't switch before 10 stacks.  Remember to give healers priority positioning on this as taking a healer out of commission for this fight means less heals for you and lots of sad panda.  After you've successfully stopped Blizzard's Big Fart Joke, you get to loot!

Viable 10 Man Loot:  Abracadaver, Kilt of Untreated Wounds, Bloodstained Surgeon's Shoulderguards

Viable 25 Man Loot:  Plague Scientist's Boots, Plaguebringer's Stained Pants, Lingering Illness

Now that you've taken out both Aboms and cleared out Putricide's creations, he's opened his door to you and invited you to come in and have a go!  So turning back into the hall and going up the newly opened middle hallway you find yourself trapped in a small room just before Putricide's lab where a swarm of bugs tries to eat you.  There's an AOE damage aura in here and some bugs that fly in and act all annoying, but it's nothing that's not easily healed through and you can just AOE the bugs down.  Then the doors open and you walk into a room where awaiting you is...

This fight is a pain in the ass.  I'm not going to lie.  But it's also pretty fun and challenging.  I have to commend Blizzard for making their wing bosses refreshing instead of ripping off mechanics from old instances.  I've also had the privilege of being the off tank for this fight and doing the Abom part of the fight.  Kudos to Blizzard for mixing it up.

When you walk into Putricide's Laboratory, you can see that the room is divided into two sides, with his alchemy table at the far end of the room where he himself is standing.  Before he's aggroed you can move about the room freely giving him a little bit of room so you don't accidentally pull.  On the right side of the room facing the table you see a green tank of ooze.  On the left side, there is an orange tank of gas.  Depending on the strategy your raid has adopted, you'll spend most of your time on one side of the room or another.

This fight has three phases.  When the main tank aggros Professor, the off tank will run up and drink a potion that turns him into an abomination.  The off tanks big job during this fight is to eat ooze pools and slow the slimes that come from the tanks.  Throughout the entire fight, Putricide will slow down green slime pools on the floor that do damage over time.  Don't stand in them.  The slimes are called Unstable Experiments and they alternate sides.  You have a few minutes of straight DPS time before Putricide summons the first slime.  It always starts on the green side.  In one strategy, the raid stays on the green side the entire time.  In the second strategy I'm familiar with, the raid switches sides to the opposite side of the current slime.  My guild does both strategies.  The most important thing is to switch your DPS to whichever slime is down no matter which strategy you use.  You have to kill them so they don't blow up the raid and you can start beating Professor down again.  The green oozes freeze a player to their spot and do AOE damage to all targets close if they reach the player.  As a mage, you can blink away from the spot and extend the distance between you and the green slime if it chooses you.  This gives you a bit more room to kill it in.  The freeze it uses does NOT stop you from casting, so you should continue DPSing as you can.  With the orange ooze, you get a slow debuff put on you.  You have to kite this ooze, so let the other DPS handle it and keep it from reaching you.  The damage the orange does if it reaches it's target is raid wide and a lot more damage.  You can blink away with this one too to give yourself more room.  Hopefully the off tank will have slowed the ooze that's up giving you a bit more time to kill them.  Ideally, they go down fast because your DPS is good and you can put more damage on Putricide before the next slime comes up.  It's also imperative not to have a slime up when Professor casts Tear Gas at 80%, stunning the entire raid and enters phase two.

Phase two is basically like phase one, save for a few extra things to dodge.  Putricide goes up to his table and drinks a potion during Tear Gas, then sprouts an extra pair of arms that throw two new obstacles in the room.  As a ranged DPS, you should really only have to worry about Malleable Goo which manifests as a bouncing green ooze that explodes and does an AOE haste debuff as well as significant damage to all that land in it's explosion range.  The other thing he throws down is Choking Gas.  This looks like an alchemy flask of green gas and is mostly thrown down at melee range.  However, if Putricide is being moved across the room, it's not a bad idea to keep an eye out for orange stuff.  Losing that much hit is a major decrease in DPS and some bad juju.  Throughout this phase, avoid the two new abilities, kill slimes, and DPS Putricide when you can.  Again you don't want a slime up during Tear Gas, which happens again at 35%.

Once Tear Gas stuns everyone again, Putricide runs up to his table and starts chugging potions.  He gets bigger and angrier.  His damage and attack speed increases by 50% and while he no longer summons slimes or tosses out goo or gas.  He still throws down the ooze pools and since your off tank has been ejected from the abom, he can't get rid of them for you anymore.  At this point, the tank will probably move Putricide to the outside of the room to avoid the ooze pools.  It's usually a good idea to stay ahead of the slime, but be careful about getting in the kiting path as you may end up a target for the ooze pools and cut off your tanks.  During this phase, Professor does a stacking debuff on the tanks that does raid damage so there's a tank switch to prevent it from getting to high too fast and making it harder on the healers.

Phase Three is usually bloodlust time so you should probably save your second set of cooldowns for this part as fast DPS is key to getting him down before he wipes the raid.  There's not much difference between the 10 and 25 man fight other than the slimes come faster and there's more people to dodge.  Once he's dead, he leaves presents behind!  Hurray!

Viable 10 Man Loot:  Pendant of Split Veins, Shoulders of Ruinous Senility, Cauterized Cord 

Viable 25 Man Loot:  Vanquisher's Mark of Sanctification, Rigormortis

Backing up a moment, we've not covered the raid weekly available for Plague wings.  

 This quest becomes available right after you enter the Upper Spire from an NPC human female named Alchemist Adrianna.  It's called Residue Rendezvous.  The nice thing about this quest is that only one person in your raid needs the debuffs in order for the entire raid to get credit.  In order to get the debuffs, a player must be hit by Slime Spray on Rotface for Green Residue and Gaseous Blight for the Orange Residue.  If you die or use an invulnerability spell (ie Iceblock, Divine Shield) it wipes the debuff.  The duration on both of them is 30 mins.  So you basically have one shot at getting this done after you kill the first abom.  I'd recommend going after Rotface first as he's the bigger pain in the ass of the two.  After doing this you get free badges, money, rep, and a shot at a random drop purple.