Anti-magic Shell (AMS) is one of the most underrated and overlooked death knight abilities we have available, given it's benefits and flexibility. When someone asks me for help with their deathknight, usually the first thing I advise them to do is put AMS on their action bar and use it at least once an encounter. Yes, AMS can be utilized in every single encounter currently in the game. Today we'll take a look at it's benefits (hidden and otherwise) as well as when to use this amazing ability. Let me be clear, this will be from a DPS point of view, however much of what I have below can be mapped over into the world (uuuhhhgggg) of deathknight tanking.
Bubble Wrap Goodness
Basically put this ability will absorb incoming magical damage and convert that into runic power. It's on a 45 second cool down and only costs 20 runic power. This means it's very cheap to use and can be used multiple times during an encounter. As a side note, I use a macro for my AMS that also includes a Fel Healthstone (aka lock candy) that way I know it gets used. AMS will absorb up to 75% of a damaging spell, up to half of your max health at the time. Interestingly enough, with the ICC buff increasing health it also increases the durability of your AMS as well. Titleist, for example, has around 42k health while raid buffed inside ICC so you can expect to absorb around 21k worth of damage. That is a fairly significant amount of damage to be avoided and can really help out a taxed healer. Now if you are an astute reader you will notice the verbiage of the ability, absorbing incoming magical damage, which is a very important aspect of this ability. [As of patch 401, you must talent into Magic Suppression for this ability to absorb and convert. When AMS absorbs damage it converts it into runic power. To be exact, 2% of the incoming damage is converted into more runic power. More runic power means more Frost Strikes. More Frost Strikes means more damage. More damage means a smile on my face and a pissed off Ret Paladin. The last benefit of AMS is it granting you immunity against the application of magical effects. In PVP this can be just as important (if not more so) than the absorption effect. So just to highlight the awesomeness of this ability; it's a defensive cool down that can be used to increase your offensive capabilities and keep your debuff bar clear.
Once You POP...
Here are some specific examples of when to utilize AMS in current content, and the reasons to do so. Understand that the use of AMS is not limited to these situations, however they are common enough to be mentioned for the raiding DK in Icecrown. I also want to note these are not considering the heroic versions of these fights, just the normal version. In some instances you can use AMS the same way in the heroic version, but not always. Some of the heroic mechanics become inst-kill and even AMS in all it's awesomeness can't save you there.
Try to avoid the coldflame the best you can, however use AMS if you are running low on runic power or cannot avoid the damage.
Stand in her Death and Decay and continue to pound her face in.
If you are running low on runic power, pop an AMS while taking a rocket to the face.
One of the few fights that is mostly physical damage. However AMS can be used while fighting the blood beasts to protect from Saurfang gaining resources if they begin to attack you. AMS is the least helpful on this fight but further within ICC this will change.
Utilize AMS and Icebound Fortitude during Fester's Pungent Blight to take virtually zero damage. Your local healer will hug you.
If you get stuck facing a slime spray while delivering your ooze to the add tank, use AMS to mitigate damage. Also if you are delivering your slime to the add tank and your half of the room suddenly fills up with slime, use AMS to get our safely.
There honestly isn't a bad time this entire fight to use AMS, given the assault of slimy magical damage from all fronts. If you want some extra face time on the orange slime you can make sure to have AMS up and active while it is select it's follow victim (that way it's not YOU). If you cannot avoid malleable goo for some reason, AMS can save your life as well.
This is another fight were you are being assaulted on all fronts from magical damage. The best example I can give is while on Taldaram. Use AMS to mitigate the empowered flame sphere as it fly's overhead. Otherwise, feel free to make liberal use of it.
AMS is ballin' during this fight. Yes, I just said ballin'. I use AMS during the air phase to survive. I use AMS during swarming shadows to survive. I use AMS during blood link to survive. Then I use all that extra runic power and my vamp buff to unload TWENTYFOUR THOUSAND damage per second into the queen herself.
I use AMS in two situations on this fight. The first priority for this should be to soak up the blazing skeletons when they lay waste. Hopefully your raid team has some outstanding dps and can bring them down without many casts, but using AMS on lay waste can really ease your already taxed healers. The second situation is when a lich aoe frost bolts the room. If you can pop off AMS prior to that you can avoid the slowing debuff which is a raid killer as well as being super fucking annoying.
I wouldn't even attempt this fight if I didn't have AMS, and that is the honest truth. That said, there are some huge differences between how the different trees use AMS during th is fight, but I'll start with frost. Being DW frost means you are attacking at a VERY high rate of speed. While this is great for damage, it's not so great for getting the stacking debuff, permeating chill. Anything over 4 stacks of this makes healing the damage unmanageable for the healers. My advice is to wait until you have 4 stacks of the chill before making it a concern, then pop AMS when you have less than 5 seconds left on your 4-stack. It will block the application of a 5th stack and allow all the stacks to fall off completely. By the time you have worked up another 4-stack it should be about air phase time, allowing the stacks to fall off without any unavoidable loss to your damage output. For unholy and blood deathknights, the basic idea is to save AMS and Icebound Fortitude for the frost blast she does after she grips the raid, just make sure to pop both and continue to dps her without running out. Unless you are working on heroic mode, you will survive with minimal damage done to you.
There is so much magical damage in this fight, you could use it every time it was up (and you just might) and it would not go to waste. My suggest is to use AMS during the outside phases when Arthas is casting pain and suffering. It will give you the runic power needed to slay those raging spirits that are popping up. Another helpful time to use your bubble is when the vile spirits are descending upon the raid in the last phase. If you cannot avoid being near them, pop AMS and maybe survive. If you do you'll have plenty of runic power to finish off the Lich King.
...You Just Can't Stop!
Well there you have it folks, how to utilize AMS to the fullest in ICC. That said there are always other ways to use it, and I'm sure I may have missed some really good situations in which to use your bubble. The aforementioned are just the ways I personally use AMS to kick some paladin ass. I hope you enjoy the mini guide.