Combating the Corruption

NPC ID's for Silver Dragon (until they add it themselves, just c+p these numbers (just the numbers) into the 'custom' field and hit 'okay')

162249 Corrupted Acolyte
162250 Corrupted Assassin
162252 Corrupted Beheader
162290 Corrupted Bloodseeker
159087 Corrupted Bonestripper
156709 Corrupted Despoiler
162246 Corrupted Dominator
162244 Corrupted Doomsayer
158632 Corrupted Fleshbeast
162247 Corrupted Jailer
162243 Corrupted Mind Eater
158531 Corrupted Neferset Guard
162254 Corrupted Observer
158706 Corrupted Putrefaction
162251 Corrupted Summoner
162253 Corrupted Thaumaturge
162241 Corrupted Thoughtstealer
162245 Corrupted Tormentor
162248 Corrupted Watcher

These NPC's can drop [currency=1719] too, so it's worth killing them whenever you see them. They also don't seem to have a daily lockout... I killed the same one twice (respawning within the 20mins or so it took me to write this up) and got another 10 memento's off of it.
EDIT: Seems there is some diminishing returns on this however, so while you'll get 10 memento's per kill to begin with, you'll get 5 and then 1 if you kill lots of them. (Thanks Buenaventura). Is still worth killing them if you come across them though imho. :)

Also note, they generate a debuff that stacks (every stack forces 1 corruption on you for half hour)... seems to be roughly every 5secs? urgh...

Happy hunting! <3

A few things things you need to know about this achieve:

1. 
   
2. You only need 10 of the corrupted NPC's. Not all of them. That's because this achieve is being nerfed in 11.07: https://www.wowhead.com/news/massive-battle-for-azeroth-achievement-nerfs-in-patch-11-0-7-351343
3. 
   
4. These corrupted NPC's have a chance to spawn straight away without any need to kill the non-corrupted versions. It's entirely RNG on the shard that you character loads into. Which means you can cheese this achieve by hopping shards.
5. 
   
6. You cannot use free accounts to create trial characters (that feature is disabled without a sub). So you will need to pay an additional sub fee to manually shard hop yourself.
7. 
   
8. Low level characters do not have automatic access to N'zoth assaults. I suspect you need to be a minimum of level 60-80 to load into the N'zoth assault version of Uldum. If you try to use a level 10 character for example, you'll just load into the Cata version of Uldum that has no BfA mobs in it and no N'zoth purple environment.
9. 
   
10. There is no point to adding any of the ID's of the corrupted NPC's to Silverdragon, Rarescanner or whatever rare finding addon you use. That's not how any of those addons work anymore (for custom npc's) due to blizz nerfing the way rare detection works years ago. These addons will only ping custom NPC's IF you mouse directly over them, thus making them useless for this purpose.
11. 
    
12. These corrupted NPC's will only spawn when the curruption zone is in the middle of Uldum or the Vale. They will not spawn when the corruption zone is in the other 2 areas (east or west) of either zone. Each zone has 3 possible N'zoth assault variants (east, middle and west).
13. 
    
14. I strongly recommend you do not use skyriding for this. You will blow right past areas of the map long before the mobs even have a chance to load into the game world.

Thus, the best way to detect these NPC's now is to fly around and spam a /tar macro like the following (you will need to use 2 macros due to the number of possible NPC's). I use these multiple macros instead of just "/tar corrupted" so that I don't kill duplicates. Just edit the macro to delete whichever mob you have previously killed. Of course, if you don't care about that, then you can go ahead and just use that macro instead of the 2 below:

NPC's that can ONLY be found in Uldum

/tar Corrupted Summoner
/tar Corrupted Thoughtstealer
/tar Corrupted Watcher
/tar Corrupted Assassin
/tar Corrupted Bloodseeker
/tar Corrupted Jailer
/tar Corrupted Neferset Guard
/tar Corrupted Tormentor
/ping [@target] Attack


NPC's that can be found in BOTH Uldum and the Vale

/tar Corrupted Acol
/tar Corrupted Behe
/tar Corrupted Bones
/tar Corrupted Domi
/tar Corrupted Flesh
/tar Corrupted Mind
/tar Corrupted Obser
/tar Corrupted Desp
/tar Corrupted Doom
/tar Corrupted Thau
/tar Corrupted Putr
/ping [@target] Attack


(I had to truncate the second macro due to the character limit, but it should still work fine.)

Doing this whilst shard hopping (whether via alt accounts, other players or simply doing it on different days or different times of the day (the zones reset roughly every couple hours) is the best way to get this achieve.

If you want to use the old method of farming mobs until you force a spawn, that's fine. You might find adding the custom NPC ID's to Silverdragon worth it if you're going to use that method. But I try to not spend hours and hours doing one achieve if I can avoid it. Also, doing it that way makes you vulnerable to other players stealing your kills since these mobs have multiple spawn points in entirely different areas of the map.

Shard hop and trust in the RNG gods is the way to go.

I don't wanna hurt any feelings, but this achievement defines perfectly what this game has become. It is insane and a non-sense being all the time doing the same: kill "N" times the mob "X" in order to get the final mob "Y". And of course everything based on RNG. It is not real progress, it's just repetition all the time. I don't want to mention the number of times I required to get a [npc=162248]. Madness..., just that.

And what's even better is that the achievement provides nothing, no mount or title or toy...

So, what's the point of playing this kind of content? Waste of time