Diplomacy, for recruiting creatures of levels 1 and 2.

I voted for diplomacy; you just need to prohibit converting joined creatures in Necropolis.

Why? You paid for them; do what you want. If Necropolis hires everyone, it will quickly run out of money. Besides, creatures of level 2 cost a lot, so only level 1 creatures should be hired (I tested: 84 skeletons cost 5000 gold, 52 Greymarch Centaurs cost 4600, 38 Grem cost 1500, 71 Phantoms cost 1500, but there aren't that many Phantoms and Imps; for example, 37 Greymarch Gnomes cost 5500), and not Necropolis creatures should buy their own troops. And also: the probability of the Diplomacy skill appearing is quite low for all heroes, except for priests, and the probability of heroes with the Diplomacy skill appearing in the taverns of Inferno and Necropolis castles is lower than for light castles.

Maybe someone knows the mechanism for calculating the cost when joining creatures, but I think my empirical data shows that diplomacy is not that cheap, and there aren't that many level 1 creatures with a low all value.