I’m not sure where they got the information that DKs can only be created by a lich. There’s plenty of evidence in-game to show that others can make them— the banshee Baroness Anastari, for example, is shown to be attempting to create a new generation of death knights in the Eastern Plaguelands (in the Caravan questline), and even the bulk of Acherus death knights are seen being resurrected by necromancers in the opening questline for the class.
I think all it takes is an exceptionally skilled necromancer to do it, because it requires not only raising something from the dead, but raising it to still have sentience, personality, and skill instead of just an angry ghoul. Liches are by default the most skilled necromancers, so they are naturally all capable, but they’re not the only ones.
That all said, there is certainly some debate on the continued creation of death knights. It’s always been a point of disagreement but flared up again with the introduction of the Pandaren DK NPC Gravewalker Gie. Was she a rare and unfortunate traveling Pandaren who got caught up in the Scourge war, or did she offer herself to the Ebon Blade some time last expansion? Dave Kosak suggests that the former is plausible.
But what about Acherus itself? While it’s easy to fob off the lack of changes on “Blizzard doesn’t care about old zones; those poor ambassadors have been touring Silvermoon for eight years” (which is entirely true), it’s just as fun to sometimes work with the given flavor in the world instead of ignore it. They’re still training disciples. There are still people within Acherus who are not fully-fledged death knights and are still learning the basic schools of magic (complete with lessons that involve summoning a completely innocent Cenarion druid and murdering her, so we can all be spared the insistence that the ziggurat’s operations are all tightly-controlled by Mograine for the sake of good morals).
Personally? I think they would allow new death knights to join their ranks, but under a few stipulations:
- -They are a “rescue”— death knights risen by the remaining Scourge forces (or unaffiliated necromancers) that have broken free and need somewhere to rehabilitate and take out their hunger without judgment. Death Knights that may have broken free upon the Lich King’s death (similar to how the original Forsaken broke free) fall under this category.
- -They willingly join, pledging themselves and following the same rules set in place for everyone else: you may stay with the Horde or Alliance and be with your people, or you can remain neutral and serve Mograine against the Scourge. Whichever you choose, you will be swiftly ended if you’re caught making trouble with Acherus’ resources.**
- (Related, alternatively) a soldier or adventurer has picked up a corrupted runeblade (one of many likely abandoned by fallen death knights across Northrend and the Plaguelands after the war) and has become irrevocably corrupted themselves. Their choice is to fall into madness or join a group capable of teaching them to harness its power. This is something of a combination of the previous two options.
- -They understand their danger and Acherus maintains a ledger of all known death knights. If you’re not in it and refuse to be, you are assumed to be Scourge or a similar threat.
- -They would not raise random dead bodies to add to their forces. This would make them no better than the Scourge.
(**This is the part where most people disagree. Storytelling-wise, I don’t -completely- agree with it myself, but it accounts for the perpetual acolytes and also opens up a lot of RP scenarios. If you must break lore, break it sensibly without shattering the foundations of the NPCs/organizations involved. People are free to disagree, but know that I’m not going to debate it. It’s something I’ve seen happen in RP for five years now and arguing its plausibility isn’t going to stop people from doing it; I’m just here to offer logical RP scenarios that don’t reach too far up the lore’s rectum.)
So— yes, I think it is possible for their ranks to grow in small numbers, but unlike the Forsaken, it’s all due to very specific and special circumstances. Amal’thazad would likely be the one responsible for any direct raising from the dead within Acherus, but not necessarily the source of every new death knight.
I see a lot of people argue that “Mograine would never allow it”, but a reminder: he set the player out on the path for Shadowmourne. He was very ready to watch someone succumb to darkness as he asked them to go harvest souls for it. I cannot imagine him personally overseeing any new addition, but should he encounter one, he’d probably give them the same warning he gave the wielder of Shadowmourne: if you falter, I will kill you.
Yeah, you can actually watch them raise DKs in the first part of the starting zone. The necromancers there don’t look like anything special:
(In other words, they’re not important enough to be named.) They are apparently working under the command of Instructor Razuvious.
Interestingly enough, between the little scene where about half the initiates are destroyed where they are created, and the player slaying an unworthy initiate in a quest, the process that they use to raise DKs seems to be pretty flaky. Most likely because they were mostly meant to be cannon fodder anyway.