Been Waiting a Long Time for This (10 player)

Don't be fooled by this achievement like we were! Necrotic Plague gains a stack each time it kills or runs out of time--NOT EVERY TIME IT JUMPS. There is a misleading debuff on the Lich King, where he will gain one stack of Plague Siphon each time Necrotic Plague jumps from one target to the next, and if you're special like I am, you might confuse 30 stacks of Plague Siphon with 30 stacks of Necrotic Plague unless some guy on Wowhead explicitly warns you otherwise.

That being said, the strategy for obtaining this achievement can be done a variety of ways, but the primary thing you need to know is this:

Get the Necrotic Plague onto the adds as late in as possible during phase 1, without pushing the Lich King into phase 2 (meaning, without getting him to 70% or lower). This means you're going to want to dispel Necrotic Plague on the first person it comes out on while they're standing away from the raid (and away from adds), thereby eradicating the first plague. You can treat the rest of the plagues like normal at that point, since the plagues--while they might initially be separate stacks--will eventually jump onto the same add and fuse their stacks. Anyway, what you have to have happen is have the plague jump from add to add (as adds continually spawn during phase 1), and avoid having to dispel it off of friendly players as much as possible, BECAUSE! each dispel removes a stack of Necrotic Plague. Additionally, each dispel will add a jump, thereby increasing the damage the Lich King does through Plague Siphon, so you definitely have motive to avoid that scenario as much as possible.

One last thing--the ranks of Necrotic Plague listed elsewhere on this page for this achievement are WRONG. The rank I listed is the one for 10 man regular and if you're pursuing the 10 man meta and aren't doing it while fighting Lich King Heroic 10, this is the accurate rank. Just my two cents.

Good luck on your drake achievements!

--Gnomaldo of Gnomeregan

Necrotic Plague was failing to stack in patch 8.1.5 (preventing soloing of this achievement) but it's been confirmed fixed in 8.3.7. If you were having issues during that time, give it a try now! Thanks to everyone who reported/confirmed!

Gonna try to simplify this solo strategy for all classes by combining info.

TL;DR: Stand on stairs, wait for him to summon 9+ Drudge Ghouls, summon yourself a helper mob to attack him so that he'll cast Necrotic Plague on you, let Necrotic Plague jump around until 30 stacks, burn him down.

SHADOWLANDS WARNING: Now that everyone is super overpowered and such, you could be too mighty depending on your class. Your helper mob could do too much damage and put Lichy into phase 2 prematurely, or your iLvl/weapon could affect plague damage to ghouls, etc., etc. So, be careful out there! Use weaker helpers if you can and don't be afraid to remove your gear if you need to. And don't forget that [spell=70337] scales to your level. Good luck!

Pre-Fight

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• Bring a helper mob. This mob will eventually be used to attack [npc=36597].
  
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  • This can be class spells like [spell=34433] or a Hunter pet.
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  • Or it's possible to use a trinket that summons a mob to fight for you, like [item=24124], though it will take quite a bit of patience if it's a proc. Just make sure it doesn't do too much damage or you'll end up in Phase 2 prematurely.
    
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    • Here's a nice list of trinkets with "summon" in the description. (FYI, [item=128318] doesn't work in raids.)
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• You can also set [npc=36597] as your focus to more easily keep an eye on his buffs, which can be helpful (as explained later).
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• I also happen to use TidyPlates and made sure debuff auras were turned on to help me more easily keep track of [spell=70337]. Not necessary, but helpful!
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• DBM will also tell you when [spell=70337] jumps, which is handy.
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FIGHT-O!

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2. Start the fight. DO NOT attack [npc=36597].
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4. Stand on the bottom few steps of the staircase, with your back to the throne. (This will prevent you from flying over the edge to your death from the shadow traps.)
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6. Let [npc=36597] summon some [npc=37695]s. Wait until you have at least three summons-worth (9+ ghouls) hanging around.
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8. Once you have enough, have your helper mob attack [npc=36597].
   
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   • If this helper is from a trinket with a proc you can't control, attack [npc=37698] to trigger the proc. DO NOT attack [npc=36597] or [npc=37695]s.
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9. 
   
10. Once your helper has aggro and [npc=36597] is attacking it, he will cast [spell=70337] on you.
    
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    • Make sure you're targettable or he won't cast it! Don't fake your own death or disappear into thin air or anything like that.
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    • Stop all damage once you're plagued. So if your helper mob is something like a Hunter pet, change it to passive at this point.
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12. Now hang out until [spell=70337] gets to 30 stacks.
    
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    • It jumps between mobs (including you) every 15 seconds. The stack count increases with every jump. After it jumps from a ghoul that ghoul will die, so don't attack them yourself and let [npc=36597] summon them all he wants.
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    • You will have to tab around the mobs to keep track how many stacks it has, which can get a bit tricky. If you have an addon that helps, great, but you can also keep an eye on [npc=36597], whom you should have focused before the fight. It's likely that his stack of [spell=74074] will be similar to the [spell=70337] stack count. It's usually one or two behind (or can be way off) so don't rely on it, but it may let you know when you're getting close.
      
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      • The plague debuff can be especially hard to find if you have a lot of ghouls. Be patient! If you're really sure that it's completely gone (either because you ran out of ghouls or you moved, etc.) then you can start over from Step 3 on the same attempt. No need to restart the fight. (Just make sure it's really gone! If you start a second [spell=70337], there's a chance you could end up without the achievement because math.)
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    • By the way, [spell=70541] can get nasty, especially if the plague jumps to you again. Try to keep yourself above 90% health while you wait.
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14. After you're sure there are at least 30 stacks of [spell=70337] (more than 30 is fine), burn down [npc=36597]. You can stop worrying about [spell=70337] now.
    
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    • Should be relatively simple to ignore the other mobs/phases/mechanics at this point if you're level 100+. Just stay on the stairs and focus DPS on him. You should be fine.
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    • If you're having trouble killing him without dying once you have the requisite number of [spell=70337] stacks, try the kiting method mentioned by liquefry to avoid plague damage. This method did not work for me -- I had to stay still -- but it may work for you if you're having trouble.
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15. 
    
16. He'll go on his 1:30+ roleplay rant and then you can get that final killing blow.
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18. Achievement's all yours!
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Hope this helps!

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Edits:
- Increased ghoul start number from 6+ to 9+ per a few replies saying 6 was too few, just in case!
- Added link to liquefry's reply (to my xpost on the 25-man version) for squishy folks who die from plague/infest damage before they can kill The Lich King.
- Added a Legion warning for higher level folks, brought to you by some helpful heroes in the comments below my x-post on 25-man version.
- There are now several comments that people were trying Touch of the Void as the summon trinket but it doesn't work in raids, so I added above as a preemptive warning to hopefully save people some frustration.

Two-manned this today on a level 90 Blood Death Knight tank (in DPS gear) and Mistweaver Monk healer, approximately ilevel 470 gear. Never been to this raid or read anything about the fights prior to tonight, and wanted to see how far we could get on the achievements for the mount.

Have the tank grab the boss and hold all of his adds near the base of the stairs. The healer can stand in the center area at nearly maximum range just to be safe. The boss will ONLY ever cast fresh stacks of Necrotic Plague on the healer, who stands far enough away so that the existing stacks on the adds will not bounce to her. The tank is only damaging the boss and the adds enough to keep aggro on them so they're nice and grouped at the stairs, and DO NOT damage the boss below 70% or he will transition into phase 2 and the adds will stop spawning.

Necrotic Plague will bounce to a nearby friendly or enemy target when it is dispelled, expires, or kills its target. If it is dispelled, its stack is reduced by 1 (unless of course there is only one stack -- it will bounce as a single stack rather than being destroyed). If it expires or kills its target, the stack increases by 1. It deals 50,000 damage every 5 seconds for 15 seconds for each stack of the debuff that is present. This means that by the time it starts to approach the number of stacks needed for this achievement, it will be dealing over a million damage per tick and will one-shot any player after 5 seconds, even at 90. (There is a possibility that future expansions will give us enough health to survive this, but I'm not expecting it any time soon.)

The first time Necrotic Plague appears, have the healer dispel it right away while standing far from the adds, so that it disappears, or just let it tick while healing through it. It took us four or five attempts to figure this one out, because many times, the plague would kill all of the ghouls before the boss summoned more of them, and then the stacks wouldn't have any more targets to jump to and would disappear entirely as soon as it's dispelled (it can't bounce to the boss himself, and this is good, because otherwise it would deal a ton of damage and we'd trigger the phase transition right away). Giving him more time to summon adds at the start of the fight helps ensure that there are enough of them to last through the end of the phase, even when the plague starts one-shotting the ghouls every 5 seconds.

Each time the stack jumps, the boss gains a +2% stacking damage buff, and he'll be dealing 60% more damage at the very least when you're done, so there's no reason to start stacking the plague again if it falls. Better to just wipe the fight and try again.

On the second Necrotic Plague, the healer runs in to the group of adds, dispels the disease, then runs back to the center of the platform. It is extremely important to watch the tank very very carefully for the rest of the phase. If the plague bounces to the tank, dispel it off immediately or it will deal massive damage and possibly one-shot the tank. If the very first one bounces from the healer to the tank, it's okay to just let it tick because it's still low enough. But once it's in the double digits, you really can't let it tick at all.

For the third Necrotic Plague, and every other one that appears on the healer from this point on, just let it tick and heal through the damage. Due to the new dispel mechanics, dispels have an 8 second cooldown. If the healer dispels the easily-ignored plague off of themselves, and the plague on the adds happens to bounce to the tank soon afterwards, that is going to result in a wipe. Best not to risk it.

Continue watching for plague on the tank and keeping both players healed up, to prevent too much damage from Infest, which is removed when the target is above 90% health.

Keep tab-targeting through the adds to keep track of how many stacks there are, and as soon as it hits 30, have the tank start damaging the boss to bring him down to 70% and trigger the phase 2 transition. The healer can use whatever ranged attacks she has at this point to pick off the weaker ghouls, to reduce their numbers and give fewer targets for the plague to keep bouncing between. Continue to be very careful about dispelling diseases off of the tank during the transition, as there will still be some adds up. It will probably jump to the tank at some point during this transition, and need to be dispelled off in order to get rid of it completely. Make sure the healer continues standing a distance away from the tank until the disease is gone -- you don't want a 35-stack plague jumping from the tank to the healer while her dispel is still on cooldown.

The damage the boss is dealing to the entire platform during this time can easily be ignored, but watch for the Ice Spheres that spawn every few seconds. If they get close enough to their target, they will explode, knocking anyone nearby off of the platform to their death. They have very little health, so a single death coil or jade lightning tick will destroy them. Make sure you stay away from the outer ring of the platform, as he will cause it to fall at the end of this transition and you'd better not be standing on it when that happens (not hard to avoid).

During the second phase, the valkyr will grab the healer (I assume the tank won't ever get grabbed, every time we did it it was the healer) so the DK should be specced into various stuns in order to slow the valkyr enough to kill it before it carries the healer off the edge to her death. This should only happen once during phase 2, as the boss will be taking enough damage to go into the next phase transition soon afterwards, which works the same way as the previous one.

Phase 3 should be quick and easy -- we brought him down to 8% before any of the Vile Spirits even showed up and before he did this whole "transporting us into Frostmourne so we have to help rescue his father" thing I was reading so much about, and ended up getting the Neck-Deep in Vile achievement as well without even trying for it. I saw the Vile Spirits floating around during the RP part of the fight, but with both of us dead, they didn't have anything to attack, and eventually they just died and fell back down to the platform.