OKAY SO LISTEN THE FUCK UP THERE ARE THREE* KINDS OF DEATH KNIGHTS
The firstity first of them were, yes, as you so eloquently put it, orc soul + human ???? success. Specifically, orc warlocks. More specifically, warlocks of Gul’dan’s Shadow Council at the formation of Doomhammer’s Horde, and they generally made shitheaded dbags of themselves during the Second War. The ones that actually still exist are in Outland, (presumably) all wandering as spirits after being slain by Teron Gorefiend, the first and now the last of them.
The Second Generation are the classic runeblade-swinging fallen paladin that usually comes to mind when picturing DKs. These are men and women (mostly humans) that swore their allegiance to the Scourge during the Third War. Most of them were paladins, but some of them were also wealthy landowners or other powerful individuals that could further the Scourge’s influence. Most of them did not technically suffer an actual “death”, but rather a slow transition into a state of undeath as they wielded their unholy magic. As I mentioned earlier, there were a rare few from that era that were killed and raised into unwilling service (Mograine, Zeliek, Thassarian, Koltira, Falric, Marwyn, etc), but the rank of death knight was typically reserved for the Lich King’s most elite and loyal.
The Third Generation DKs are the ones introduced in the Wrath storyline (although as I went into earlier, the distinction is actually probably more in the magic type than it is the point of creation). While their degree of undeath can vary from “pale but alive-looking” to “someone get the duct tape”, all of these variety death knights were very much killed and risen into unwilling service. Their service was ensured twofold: by their bodies and minds being overcome by wracking pains and agony unless they vent that bloodlust, and by their soul-eating runeblades (don’t get me started on runeblades and their sketch-ass lore) constantly demanding to be fed.
They were significantly less picky about whom was risen in this manner, but they still had some standards— these were still elite shock troops meant to cause heavy damage. Paladins, soldiers, and other combat-strong bodies and minds were favored.
* YEAH BET YOU FORGOT ABOUT THAT ASTERISK
I put that there because there are these obnoxious motherfuckers called Worgen death knights and they’re a thing and everybody groaned at them being a thing but boy are they a thing. Their story does not easily fit into the Acherus narrative. Most Worgen DK RPers (that don’t have an age of 8927 years old and an MRP with Breaking Benjamin theme music) will say that their character was a member (or victim) of the Scourge’s Wolfcult in Northrend and was risen up there, then either making their way to Acherus before Light’s Hope or finding freedom in some other way. Alternatively (and I think this is the actual Acherus narrative? I haven’t made a worgen DK oops) they could have been a Silverpine worgen. They are still all technically Third Gen, just very confused.
** REBLOGGING TO ADD MYSTERY ASTERISK THAT DIDN’T EXIST
There is, fanon-wise, a “fourth generation” of post-scourge death knights that I feel are worth mentioning. They aren’t acknowledged in any canon sources, but this hasn’t stopped plenty of RPers from having their characters die in roleplay and be resurrected by fellow DK RPers into undeath themselves. There is honestly no evidence that the player-type DKs are capable of raising a being as powerful as a new DK, and the Ebon Blade would most certainly be averse to creating more like themselves. However, it can still make for some interesting RP plot, and sometimes for a brand new character a post-Scourge backstory can be easier to explain, since it’s been so long since Wrath.
So while they’re not an official thing, they’re so prevalent in roleplay that I feel like, on some level, they should count.