Notably: Let's consider cases where a different outcome was possible with some degree of probability. i.e no cases of "What if Malcador had decided to turn to chaos 1!1".
I weigh this also from the POV of multiple counterfactuals. Namely, impact should not be weighed just by saying "if only X had remained loyal", but also "If X had turned and Z had turned traitor, would the overall outcome have changed?"
Most "important":
Traitor Primarchs:
- Magnus
+ If Magnus had not destroyed the Imperial webway, then the Emperor would have Won (Capital W). Hands down.
+ Similarly, if his shards had all been recovered, and he had somehow returned to the Emperor's side, even without his legion, he could have replaced the Emps and Malcador on the Golden throne, both during the siege and in 40K - another major game changer.
He also essentially wanted to be loyal, right up until he was broken, and had most of his soul replaced by Tzeentch, as the Crimson King.
- Horus
+ It's called the Horus Heresy for a reason
. - Fulgrim
+ Thanks to the Eldar's warnings, he had the opportunity to devastate Horus's fleet, and to warn the emperor in advance of Isstvan of the heresy and Horus's corruption. He could have effectively prevented Isstvan (III and IV), effectively kneecapping the entire Heresy.
+ Has the best record of any of the primarchs when it comes to actually killing a Primarch (Ferrus and Guilliman - the latter arguably being a major reason for 40K being the way it is).
+ Even after meeting Horus - he was given the "keyed" anathame - a second "heel turn" would have meant that he could have effectively guaranteed killing Horus in a duel, given the Kinebracht anathame blade that Horus gave him.
- Most of his legion were tainted by the time of Isstvan, but he was actively encouraging it shortly beforehand (e.g. the chemicals, and killing Lord commander Vespasian (equivalent rank to Eidolon, and whose forces were loyalists). So in terms of military power he'd not add as much as some, but before they went nuts, they had a very rigid command structure, so it's very possible he could have reversed much of the rot.
- Perturabo
+ One of the only non "crazy" Primarchs. Horus's effective, controlled military forces were effectively the Sons of Horus, the Dark Mechanicum, some traitor imperial guard & fleet elements, Alpha legion [small] and and the Iron Warriors. Everyone else was pursuing their own agenda by the time of the siege to varying degrees. Horus explicitly stated how much he relied on him (albeit, it might have also been affected by Pert's absurdly touchy ego).
+ Without Perturabo as the unstoppable force to overcome Dorn's "Immovable object", it's very doubtful that the traitor forces would have had the tactics to overcome the Imperial palaces defences effectively.
+ The overall record would seem to suggest that Pert is better than Dorn. He overcame the defences in the siege (it's much easier to defend at static position than to attack, GW numerical and historical illiteracy be damned), and he crushed Dorn in the Iron cage.
+ He was also essential to wrangling in Angron for the siege, with all his world eaters.
+ He also had the "uber" siege weapons from the Lion, which were said to be essential for cracking the palace.
Effectively, without him, Horus's conventional military efforts would have been crippled.
He was also shown to be extremely loyal in his personality, albeit bitter and peevish. The contradiction being him going on about how he would stay fanatically loyal, after having broken his oath before - but still, it's easy to see him continuing to fight on one side and not the other (which can be said for all Space marines overall).
Loyal Primarchs:
- Guilliman
+ Without the Ultramarines, Horus would have had a good chance of winning the siege, given the effect on morale (no coming reinforcements), and the numerical benefit of having roughly 2.5 entire legions.
However, it's very hard to see a scenario in which Guilliman would have turned. He lacked the ambition, or personality flaws. Even Imperium secundus wasn't an argument in favor of this. Still, the point stands.
- Dorn
+ Lead the defence of the palace and its fortifications.
+ One of the only unbroken legions (no Isstvan or purgings).
As with Bobby-G, it's hard to see a scenario of him being turned.
The Best of the Rest :
- Sanguinius:
Controversial, I know
. His defense of the palace & Ka'bandha is definitely in everyone's favorite moments. However, it's unclear how irreplacable he would have been. Similarly, his great "Fate" of being killed by Horus, and leading the Emperor go into a Super Saiyan 2 blood rage seems replaceable. One can easily see the same outcome with any of the other primarchs instead of him, or indeed possibly a perpetual or Malcador.
- Lorgar:
+ Second most numeric legion, and had a massive fleet.
+ Influential in corrupting the other legions with lodges and chaplains.
- If he had died and been "replaced" by the leadership of Kor Phaeron and Erebus, literally nothing in the Heresy would have changed (except for Angron not being turned into an overgrown Bloodthirster).
It might have been possible for him not to turn, if he had just smushed Erebus & Kor upon learning about their cults, and/or those two not revealing it to him if the Gods knew that would happen, so this isn't that unlikely a scenario.
In terms of military effects - the biggest thing he did was Calth and the shadow crusade = delaying Guilliman. (Not by enough, but also by just enough to be too late for the siege).
As for the rest: I would argue that while any of them turning or not turning would have had a clear benefit in terms of available forces, it would not have been enough to unilaterally change the outcome. Especially if we suppose a 1:1 exchange.
Thoughts? Flames about the ranking? Discuss!
