Long live the queen !
In the interlude before this mission, you may get the choice between keeping Cora or Scarlett, if you had her captured in chapter 5 and if you had not chosen "whispers in the dark" (if you did, Cora will kill Scarlett while sucking out her powers). I went with Cora.
Obliterating hundreds of undeads in chapter 6 with the terror ships paid off : the scared undead proposed an alliance to Gardon.
More seriously, you begin the mission in that old base from book 1 chapter 5, but this time you have undead allies to the East. They will sometimes deal with a few attackers, sometimes send an attack wave, and from time to time Saphira Whisperwind will give you a Frost Wyrm.
As is usually the case with allies in W3 campaigns, their attacks are rather weak, but can still be useful to relieve the pressure on you, or to synchronize with to overwhelm enemy defenders.
You get 3 heroes to play with - Gardon, Brian and the one you chose to replace Renova as elven queen, either Larine or Salana.
A hint tells you soon into the game that the knights of Come will kome... mmm. The knights of Kome will come if you destroy the western elven base. This is essential.
I reloaded a save once because the first time I was too passive and thus made things more complicated for myself. Acting swiftly is paramount.
At the start, I made the usual things : regroup weak units in appropriate control group, send (many) more workers to collect wood, start upgrades, chose a leadership bonus.
I went with the direwolves, because other melee units available at the beginning at terrible. Later in the mission though, they are overshadowed by the knights of Kome (they don't join you as a separate allie, they are directly under your control and you can train more).
I built a few towers around to defend, but unlike chapter 5 of book 1, just trying to set up a strong defense is not a great strategy.
You receives attacks from several enemy bases : golden guard, red guard, cyan elves from the west with a starfall hero, and pink Riana monstrosities. With 3 heroes and a few units I could repel them without too much trouble. I trained some banishers to heal my units with heal wave between (and sometimes during) battle.
A good strategy is to build up a full-food army quickly and to go along the undead when they attack the cyan base. It will be overwhelmed (especially if you can Starfall it with Salana) and once the castle is destroyed you get Cora, knights of Kome and a bunch of weak golden guards units, plus peasants to build your own extension there.
However, while I tried this before my reload, I messed up afterwards and on my successful try actually used a different strategy.
The eastern yellow golden guard base, headed by Captain Ironside, will switch side after some time in the game. This side switching will always happen, and that's why a pure defensive strategy is a waste.
With the red guard to the north and the western golden guard base of Pureheart, this new allied base is very vulnerable, and it requires your active support to stay alive.
The road towards it is polluted by assassins of the queen, ranged units which are invisible until they attack.
Later during the mission, I used the Frost Wyrms sent by Saphira to destroy the towers guardings their halls and then destroy the halls where those pests are trained (which are located north of the river, south-west of the western golden guard base). A few dragonhawk rider also help to speed it up.
But when Ironside revolted, I was still far from having done this and had to clear them. Anticipating the revolt can be useful.
When arriving towards Ironside's base, I had Salana with me and wanted to send her towards Lera the huntress, which I had spared in book 1. When Lera see Salana, she switches side too.
Indeed, the army of Ironside quickly go to attack Lera's eastern red guard base and can hugely damage it while getting killed and having less strength to defend to the west.
I got lucky and Lera was almost at the entrance of the two golden guard bases so she switched sides very soon.
However, if you had not saved Lera, or have Larine as a queen, the general strategies still apply. A few months back, I completed this mission when upkeep was more damaging to income and with Lera dead and thus never switching side.
With my troops, I eliminated the destructors which dealt too much damage to Ironside's base, then pulled back.
The plan is to get your heroes as permanent defender of Ironside's base, and to have a few peasants of your own in it to build some unit training buildings. However, I still had to deal with the cyan base.
Lera was under my control as she switched sides, and she's a really overpowered hero.
Destroying the cyan base was thus made easier (I had to retrain many units which had died in the first attempt to help Ironside, however).
Once Cora joined, I made a group with her, Salana and Lera and sent them directly to Ironside's base. I destroyed more buildings of the cyan base, began to build my own, and sent knights of Kome along with Gardon and Brian towards Ironside too.
The extension to the west is really safe (I never got attacked there, though I built a few towers by habit), so it is basically a free gold income source. Once they finished off all buildings, the weaker units got sent to die in battle too.
As I was dealing with the cyan base, things went dire for the new allies : after having done a suicide attack against Riana, the renegade red guard had not enough troops to withstand the counter-attack. Ironside's base was doing a little better, but it was a matter of time before it would have crumbled without my help.
The group of the 3 girl heroes went as fast as possible to the renegarde red guard base. All three have a OP "deal damage to all enemies in a big zone" ultimate, so I tried to cycle them. Add in their lightning abilities which deal good damage, and with decent micro they can obliterate big armies.
Almost all the renegade red guard got destroyed in my game, but with their castle saved their workers slowly rebuilt things afterwards. The AI seems to get dumber after this however, and was more useful as a defensive buffer than as an attacking asset.
Meanwhile, Gardon, Brian and other units defended Ironside's base.
I slowed down pace to take out all the training halls of the bastards assassin and to build barracks and an arcane sanctum in Ironside's base. Having a few banishers at hand proved very useful, because both of my groups received a lot of damage (in the girls group, Lera is incredibly bulky, but the same can't be said of Salana or Cora).
The next step was to eliminate Pureheart's base.
I didn't try to get some fancy unit compositions, having 5 heroes is already micromanagement hells and fancy unit compositions would have only compounded the issue. I simply went with mass knights of Kome plus many banishers to heal everybody.
To not abandon the renegade red guard base during the attack, I actually had some towers there, plus barracks and arcane sanctums which were my unit production hub.
When you attack Pureheart's base, the red guard and Riana will send reinforcements.
However, between Gardon's War Aura making all of your units dealing very strong damage and some starfall/flood, I managed to deal enormous damage. I had to constantly click between the heal wave icon and some of my wounded units (the keyboard shortcut didn't work) but despite this annoyance, It helped a lot to keep my units alive longer and destroy a lot.
I had to pull back once nonetheless, as I had lost several units, other were wounded, and more critically Riana's chosen were starting to destroy my buildings next to the renegade red guard.
Once the attack got repulsed, the deads got replaced and everybody got healed, I attacked again.
As about half of the enemy base had already been destroyed the first time, and Pureheart troops were weaker, there was no major issue.
At this stage, taking out the final red guard base and then Riana's pink base is a formality, even without Lera.
Before finishing the chapter though, there are a few things to do. First, you can get a tome of agility and a tome of force in spots once occupied by forest trolls and now guarded by queens assassins, in the forest east of your main base. The agility tome should go to Salana, the force one to Gardon or Brian.
Second, you can buy orbs of corruption in the base of your undead allies (I actually only thought of it and applied it in chapter 11-A, but it's better to do it right now). Orbs of fire are terrible, orbs of corruption are awesome. The -5 armor they inflict on attacked units is easily worth +15 or +20 damages in the later stages of the campaign. (Admittedly you could do this earlier in the mission, but it requires time to go there which you don't have much)
Third, you want to make sure that Salana/Larine has a lot of good items, and no weak one. The goal is to get all the best items of the campaign for your heroes towards the end, and if you don't give them to her now, you risk having to throw away some items better than what she will have. In case of doubt, you could give her two orbs of corruption, with one to redistribute to another hero later.
Once the mad sorceress Riana has been put down, you can enjoy the outro cutscene.