Diplomacy, for recruiting creatures of levels 1 and 2.

Joining creatures of levels 1 and 2 can make the game easier for Necropolis and Inferno, which I think is obvious and doesn't need further explanation. But if you prohibit experts from obtaining the diploma until the last level, as with the Necromancer or Vidomina, then there will be no cheap joins (there will be a negligible percentage), but you can freely spend money. Although Necromancer is still overpowered in any case. However, if you give Diplomacy an achievement, the ability to escape even from shackles, then it will become a quite interesting skill, even without joins.