"Necropolis" - changes to creatures of levels 6-7.

Well, they aren't twice as weak as, say, the same Plate Mail units. Cerberuses have similar stats.
I'll think about the defense. I think we can find a middle ground.

I didn't say they should be twice as weak; I was talking about how incorporeality/ethereal form is almost equivalent to doubling their HP. Plate Mail units and Cerberuses aren't defenseless; they have their own perks (a whole set), and the latter also have speed and initiative. Ghosts only have incorporeality as a significant advantage.

EVERYONE runs away – this is a real problem. Maybe the chance of the ability triggering could be made dependent on the ratio of the number of attackers to the number of Messengers themselves, because besides Ossir and Sinister, everyone else is weak against them in a duel.

The ability isn't being adjusted. It can rightfully be called one of the strongest abilities, after Seduction and, perhaps, Incorporeality, of all the abilities in the original game.

in general, summarizing what was said above, I think it should have been increased by 5-7 times, not 10. 10 Banshees deal 1000 damage in the first row with 0 Defense. I let them attack the Seraphim right in front of him, so you can imagine – without taking his defense into account, they dealt damage to him, and without a counterattack... (and now remember that there are three of them).

Well, that's your problem if you let a full stack of Banshees attack a unit with high defense and low HP. It's like standing in front of a dragon's breath. You shouldn't turn the ability into something useless and weaken it. It shouldn't be weaker than a direct attack on a single target; otherwise, people won't be afraid of it, and it won't be a significant feature of the unit. For comparison, the Red Dragons have a similar ability that deals 1.5 times more damage with the same no-counterattack effect, but it's unlikely to be considered cool because it's performed from a standstill, and you have to really try to miss it.

And you don't need to constantly talk about 3; in practice, you probably won't be able to build an outpost in a reasonable amount of time. It's already a problem for the Necropolis to build a simple level 7 structure, let alone upgrade and build an outpost...

EVERYONE runs away – this is a real problem. Maybe the chance of the ability triggering could be made dependent on the ratio of the number of attackers to the number of Messengers themselves, because besides Ossir and Sinister, everyone else is weak against them in a duel.

In duels, the weak dragons were effectively replaced by strong units in a 1-to-1 ratio, hence the result. If you want to balance duels, you need to reduce the number of deaths in the presets.