RTF is a global modification that replaces all towns in the game.

I think the very first skeletons were better.

Indeed... yes, probably that version was the most enjoyable to play. I didn't notice a significant difference in resurrecting Tier 1 or Tier 4 creatures...

Finally, I found a useful article about necromancy and dark energy points on the English wiki. It turns out I was wrong - the number of skeletons does increase, but it's not multiplied by the level difference; it's multiplied by the ratio of HEALTH, so a bulky Assassin for the first level was resurrected not by 20%, but by 35% of the number, and fewer Death Knights were resurrected from weak witches.

In principle, you can reduce the amount of dark energy at the start and from the necromancer's pillar, and increase the points gained from the level.