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Creatures join if:
1) the enemy army has the same unit (even if it's just one) + the hero has a strong army...
2) if he has Diplomacy...
3) if the hero is leveled up, has a strong army + a unit + a lot of money - this often happens... but if there's little money, they probably won't join...
4) ... if the neutral creature has the "Submissive" checkbox checked...
In this situation, I am very puzzled... even very much so. I checked Kosti's calculations - everything is correct... I am VERY puzzled where 53 wyverns came from... even if there was a Legion Head (and there isn't one), there would be +6 or 7... but still not 53
Added 10 minutes later
a little more for the tournament organizers... the map is 100% random... + there are so many free items on it that if someone picks them up, they will have an unreal advantage... and also... as far as I know, in the rules of online tournaments, monsters are always set so that they don't join, because that's also not good... ;);)
Dadge didn't have a very strong army or Diplomacy, and the wyverns were in an aggressive state!!! The map is random, but it has been completely reworked, and if there is an artifact somewhere that is not completely hidden by a neutral creature, then I apologize, it is impossible to notice everything (there are also unfinished things).