I decided to replay the second mission after Remix's streams in order to find new strats. Before playing, I checked the distance from the portal near the Necropolis city to the portal leading to the Overlord and discovered that you can completely avoid fighting the blue demons if you have certain skills / artifacts; Sigrif can simply run straight to the Overlord [blue demons cannot pass through there, nor can they go past garrisons, toward the utopia or onto the southern plateau guarded by death knights]. Thanks to this, you don't need to spend 1-3 months accumulating fodder to kill the blues and then Kha-Beleth. Nevertheless, you will still have to build up an army using Summon Reinforcements, as the demons available initially—including a couple of caravans to the Necropolis—are far too few for combat.
I'll briefly describe the Walkthrough for 4/4/1 here and attach several replays
Almost all chests are taken for gold to build markets and town halls as quickly as possible. With good luck, by the end of the month, it's possible to build almost all creature dwellings in all 5 castles. "Almost," because there is no need to build them all within such a timeframe; regardless of income, there is never enough money to hire and upgrade all troops in this mission. I maxed out the starting city (Ur-Hinnokh) and Thor Thrall
The start of the mission is standard—restart until Sigrif gets Chaos + Wizardry / Lore on the first level-ups, and a boost to Magic own 2nd-3rd level; on the surface—until there are at least 1-2 stacks of creeps that can be killed with minimal losses. The fights for Sigrif are simple, and ~90% of them go like this: imps use summon in the enemy's face, Sigrif hits them with chain/wave, then kites if necessary. Chaos is mandatory in the first stage, otherwise the necromancer will wipe out almost the entire army; it's needed to use Devil's Strike in this battle, which deals more damage than wave and chain once you have Advanced Chaos magic. It's also worth noting that the necromancer casts Magic Fist when he has a large army, but when it is significantly smaller, he starts ressurecting them. My best combat result is in the screenshot
m2nekr.sav
Skills: Expert Chaos with Pyrokinesis, Defense with BDP, and Wizardry. Without BDP and Pyrokinesis, it was a hopeless loss regardless of where I placed the gates.
+ fight with phoenixes for the Sar-Issa Robe
m2fen.sav I remembered and saved this because I restarted here 8 times
Capture the Necropolis and surroundings, and in the demon lands, go to the mentor, remove Chaos and take Lore with Scholar and Darkness; then get the necessary Haste, Berserk, and Subjugation from allied demons. Follow the plot, obtain the Staff of the Underworld, and then using chain/wave, a Secret crystal, and sometimes Darkness magic, clear everything around. Examples of battles where only the crystal + wave/chain were used
m2cyclops.sav m2manticore.sav Darkness is truly needed, as random stacks of shooters/demons still exist, and fire-breathing stacks aren't broken by the crystal. Sigrif is level 36 in the screenshot thanks to two very long fights against dragon legions [one stack of succubi + secret crystal and hell wave

]
Once finished, go back to the mentor and remove magic—you won't have time to use it in the fight with Kha-Beleth anyway, and his magic resistance is very high. I took Lore + Attack/Defense/Leadership/Luck.
Perform the ritual, and this is where the differences from a standard Walkthrough begin. I didn't wait for all blue demons to be at their middle/lower city; instead, I ran to the well in the Necropolis lands; by this time, the starting city was fully built. The army replenishment method is as follows: equip all possible artifacts for Knowledge, approach the well

cast "Summon Reinforcements" and use all mana to take troops from Ur-Hinnokh starting from tier 6-7. Immediately after casting, drink from the well and repeat. One week was spent gathering the army shown in the replay below.
The second difference is running from portal to portal without fighting [no Logistics skill]. At the beginning/end of the week, put on red boots, enter the stables, then stand exactly in front of the portal

And when all movement points are restored, gallop toward the Overlord. If there are no enemy heroes on the path, there are enough movement points for this maneuver. Tests with log/boots/stables performed before the run:
Header
1) No Log
Short by 12 cells
Boots / stables: short by 6 cells
Boots + stables: passes
2) Basic Log
Short by 10 cells
Boots / stables: short by 3 cells
3) Advanced Log
Short by 7 cells
Boots / stables: passes
4) Expert Log
Short by 5 cells
Boots / stables: passesThe fight with the Overlord
m2beleth.sav is not particularly difficult. The Dwarven Hammer, like the Ice Shield, is not mandatory due to the massive army. Build+artifacts

P.S. After writing the post, I realized that with such an army taken from Ur-Hinnokh (120 devils, 200 pits, etc.), you could easily wipe out enemy heroes immediately [just don't remove Darkness magic beforehand], rather than doing portal gymnastics and rushing to the Overlord, but I was going for the minimum number of days, so I didn't think about it.The strat of capturing the upper blue city followed by retreating to the utopia and summoning reinforcements from Hrothtar is slow. Even if you buy up all troops on the surface and send them via caravans to Hrothtar, you first have to wait for a moment when none of the demons with huge armies are near the portal and upper city, and it's unknown when that will happen. If you take troops from Ur-Hinnokh, such problems do not arise.On the surface, Grol usually starts best thanks to Chaos. Nimus can then be made the main using mentorship and many spawning dwarf stacks capturing mines via quests. I couldn't come up with a stable tactic for starting fights here; I just leveled Chaos, War Machines, and Log (I also wanted Defense with BDP, but it never dropped, so I took Darkness at level 14), and in some places, I either killed everyone immediately or used tricks. After getting Haste, Berserk, and Subjugation, the strategy obviously changes. Alternating red and green boots, and ignoring some neutral stacks, using Log with Path of War, I reached the most distant city (Thor Mirdala) by the end of the 3rd month.
The combat isn't very interesting; I only saved the finals with Duncan and Faedan
m2duncan.sav m2heam.sav Since ~60% of the army was given to Sigrif, I had to defend in the city. I used the "occupied" tactic—when the opponent's catapult is destroyed and all gates are placed inside the garrison, as a result, enemy flyers cannot fly over the walls
Build+artifacts


It's also beatable with the Staff of Sar-Issa, but Tactics is needed; otherwise, dragons/angels can fly straight into the city.
Overall, I enjoyed completing it with this kind of rush in 4 months, but I wouldn't say it was easy. I rolled for a good start for about 30 minutes, there were hassles bringing the army to the starting city, summoning reinforcements isn't fast, and many battles required multiple restarts, including the final one with Duncan.

If fighting with full armies, the battle with the Overlord is won in 1.5-2 turns
m2belethful.sav, and you can fight Duncan and Faedan in an open field
m2duncanfull.sav m2heamfull.sav
Battles on the 3rd week of the 6th month; Nimus has a slightly different build here to avoid losing on the first turn
Tactics is a must-have, otherwise paladins with angels and dragons will wipe out half the army immediately, and there's no saving it then