Diplomacy, for recruiting creatures of levels 1 and 2.

how early?

The lower the chance of diplomacy appearing, the higher the chance it will appear later. I disagree about the Necropolis; we need to analyze game statistics to judge. In my opinion, the Necropolis won't benefit much from this, as I explained earlier. Below, I've provided tables in a clear format showing the probabilities of heroes gaining secondary skills.