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Yeti are impossible because they are in Cronwerk. And they are not demons.
For level 2, I suggest replacing the Necromancers with Ice Golems. The Necromancy skill will be replaced with "Blacksmith." Some of the units of Tartarus kill the enemy by turning them into ice. With the "Blacksmith" skill, the hero can forge some Ice Golems (using the Necromancy dialogue window). Also, with this skill, artifacts can be forged by sacrificing experience. (when crossing the lower level limit, a random primary and secondary skill is lost).
Buildings (from Mugais):
1. Cauldron of Winter
2. Freezing Tower
3. Cliff of Fury
4. Ice Hole
5. Altar of the Polar Star
6. Portal
7. Hall of Cold - Hall of Ancient Cold
8. Hall of Absolute Cold
Grail
Absolute Zero Laboratory
Appearance: An ice dodecahedron, hovering not far above the ground.
Units:
1. Mephit - Quasi
Luck - it's cheating, the Devils have this ability. Let them then "have a chance" or "gain a skill with experience."
2. Ice Golem
(all humanoids brought to the city are transformed into them)
4. Whirlwinds
5. Ice Demons (upgraded ones shoot needles)
6. Four-winged Ice Serpents,
7. let it be the same, but call it something else.
I suggest Gelugon as the first commander.
Added 10 minutes later:
or like this: level 6 - Ice Demons, level 5 - Gelugons.
And the serpents (graphics like the Amphisbaena from the Docks, but with 4 wings) - as a summoned unit. Before the battle, a ritual is performed, which consumes: movement points, resources, mana, maybe - experience?