This is good, but I have problems with it.
Some good constructive criticism is always appreciated. Even if we don't agree, it still forces us to rethink what we wrote and that is always helpful.
I don't feel like it's enough to just have the Cabal, White Scars and regular humans as the invaders. It's simply not enough force. The majority of the Imperial forces in the Webway were Mechanicum battle servitors, tech-thralls or the Legio Cybernetica. The Mechanicum had a massive presence there and probably did most of the actual fighting. The Custodes and Sisters of Silence were a relatively minor force (who played a large role) comparatively and they only got engaged when it was necessary. The Legio Ignatum were also present in almost their entirety. So, at least for me, the attackers have nowhere near enough strength to even get to Calastar let alone make it a close enough fight that necessitates the Emperor's intervention. Even the militia becoming spawn doesn't alleviate this as in Inferno a single Custodian held a street for 9 hours straight against chaos spawn and basically made a barricade out of their bodies.
Hm. I thought it was just Custodes and some Sisters, until the daemons became such a threat that other forces, such as the Mechanicum and the Legio, were brought in. Otherwise the only Mechanicum presence was worker-based, which could still be a viable threat in battle. If I'm wrong in that, I'll definitely need to approach it again. Do you remember specific parts that stated the pre-reinforcement order of battle?
That said, you bring up a point that I should have clarified the traitor order of battle better. In my head, there is the Legiones Astartes and the Imperialis Auxilia. That's it. If it's not a Space Marine, it's under the Imperial Army umbrella. So when I say "a significant portion of the Lion's mortal forces," I'm picturing a few million conscripts bolstered by a couple hundred elite regiments, at least two or three Knight Houses or maybe a collection of Freeblades, perhaps a demi-legio of Titans, so on and so forth. Maybe a tenth of the kind of forces the Lion must have under his name, which itself would be but a fraction that Guilliman commands.
Much like Horus, neither Guilliman or the Lion commanded just Space Marines, but held authority over significant elements of every type of Imperial there is. Perhaps the name Caerulea Militia was misleading and it implies only the chaff, when I was intending it to be the name for the Lion's entire militant force. I'll make some edits down below.
Furthermore, for me this feels kinda shoehorned in. There isn't the feeling of desperate battle and the ever decreasing chance of the Imperial forces succeeding. For all the problems I have with Master of Mankind, ADB achieves that incredibly well.
Well, it kind of is shoehorned in, but I certainly want it to feel like a natural element of the narrative. While I wouldn't dare to to say that I can evoke a similar feeling in my writing as AD-B does, it's also kinda that it's just a different battle. They may take place in the same location, but they're two very different battles and that's intended.
In canon, the Webway is done. There's nothing that can be salvaged of that situation, and the Emperor's presence is absolutely needed in order for the disaster not to spread to Terra itself.
Here is an entirely different beast. It's not a war fought for years, but weeks at most. It's not a battle that poses an existential threat to the Emperor or Terra, it's actually meant as a diversion. The Emperor's intervention isn't a necessity, it's just part of the overall narrative for the Lion's failed assault on Terra. Though a doomed venture, it roused the Emperor to the threat posed by the traitorous forces, the Emperor is introduced to the battlefield, ends the fighting on Terra itself and goes down to the Webway to finish the job.
But it's the fact that there is a battle being waged above, and that the Imperial defenders have not received reinforcements, that allows for a significant force, bolstered by an entire Legion, to slowly put them on the back-foot.
For me, it would be better having this occur simultaneously with the Emperor re-taking Molech (If that's still happening, it been a while since I've checked). That way there's a reason for the defences of Calastar being weaker. You could even have the White Scars be unmolested by Imperial Forces until they get in range of Calastar bearing in mind that in this situation the Custodes still have the ability to re-take the lost ground in the Webway easily. Meaning that they don't need to waste their forces unnecessarily.
By the time of Molech, the Webway and Calastar (thanks for reminding me of its name) would include the Space Wolves, Russ, the Thousand Sons and Magnus among the defenders.
But, if we did go that route, we'd need a reason for it to be attacked in the first place. If it's not part of the Lion's assault, then either the Khan is doing on his own or Guilliman has commanded him to do so. So either the Khan needs a reason to attack Terra through the webway, or to bend the knee before Guilliman.
Also, it's likely that I've simply missed the update, but why is the Khan working with the Lion. As per the original ideas, the Scars were fighting against traitor forces when they didn't receive help from Fulgrim. It seems unlikely that they would throw their lot in (temporarily) with the traitors after that. An interesting idea though is what if Fulgrim didn't help the Scars because he both didn't trust them enough, but also the Khan insulted him at Ullanor (from Scars)and he holds a grudge because of that?
Yes, there was an update. It's in response to other criticism that felt the traitor side was missing too much force to be an equal threat to canon. It was all very mathematically deduced and the logic was sound, though a lot of elements were not taken into account. Understandable, can't expect everyone to read through the entire thing. But it did lead to the thought that maybe the Lion needed more, and from where could we look to give him more strength. Well, with relative minimal change, you could give him the White Scars.
All it would require is a reversal of the meetings to the Scars and Fulgrim, and the Scars and the Lion.
If the White Scars meet Fulgrim and the Emperor's Children first, there's still an opportunity for mistrust to become hostility and finally outright violence. Canon Khan was already skeptical of the Emperor's designs, and had a greater affinity with many of those who had turned against the Emperor. This attitude can be used against him here, if the Emperor's Children encounter the White Scars and demand to know their allegiance. The Khan, who true to his self is probably completely unaware of all that has happened, would likely not give a clear answer. He would want to see and hear for himself from those who have supposedly betrayed the Emperor what actually happened.
The Emperor's Children had just taken a beating at Miral. Men they once called brothers had turned upon them, and Fulgrim's heart would still be heavy after the death of his closest brother of all, Ferrus Manus. If the Khan cannot answer in the affirmative, then it will be assumed in the negative. No more chances. Now, it won't be a fight to the death, and I'd see the White Scars being the ones to retreat and from the get-go. But such an attack would seal the White Scars' fates. A legion whose loyalties have been confirmed beyond all doubt fought a battle against them, and that news will spread. Though it may be far from the truth, in the eyes of all Imperials the White Scars are traitors.
Much like the canon Thousand Sons, who never truly intended to betray the Emperor, that decision had been taken from them. And once done, they will have no choice but to fight alongside the traitors. It won't be hard for the Lion to convince the Khan to join him. But this is still the legion and the primarch who held a healthy fear and distrust of the warp and its corruption, and nothing they had done up to that point will have let corruption already sink in like with the canon Thousand Sons. So while they may swear themselves to the Lion's cause, the events within the webway will show them a different side to the traitors than they are willing to accept. There will be plenty of room still for the Khan to denounce both the Imperials and the rebels, and declare himself and his sons only to their own freedom and survival.
Ultimately, giving the Lion just a single additional legion and primarch does enough to tip the scales more evenly, I feel. I don't believe that Guilliman's own side needs anything else.
In point of fact, when I take FW's legion sizes as my base, use their depiction of Isstvan losses as a template for the kind of losses seen at Miral, remove the loyalist First Legion, Iron Hands, Space Wolves and Thousand Sons as anything but token players, Guilliman's side gains the edge in numbers, purely on the Legiones Astartes side. By the time the Emperor invades Ultramar, the legions at his command may have numbered anywhere from 750,000 to 1,000,000 strong, rounded. Guilliman, on the other hand, may have had anywhere from 850,000 to 1,200,000 legionaries, again rounded.
Considering the impact that the Carrion Edict must have had, Guilliman may have been able to field up to half again as many Space Marines as the Emperor could. Guilliman also being true to himself, the average Ultramar world and soldier would have an edge on the Imperial opposite. Granted, not every world or regiment of Guilliman's grossly expanded Ultramar Empire is going to be as great or powerful as those of the Five Hundred Worlds, but there'd still be an impact.
I honestly think that we've done enough with Guilliman's side of things that, when you include the Ruinstorm and overflowing daemonic hordes, you need the Emperor to personally take part in order for the Imperials to have a chance for anything other than outright defeat.