Since patch 1.6 (and 2.1 for the expansion) significantly overhauled mechanics like Necromancy and Training, players rely on "papers" or guides to plan their hero's development toward "Ultimate" abilities. The Skill Wheel System
The game groups the 16 secondary skills into a circular formation. You start with one or two skills based on your faction and hero class. From there, you can only learn skills that are "adjacent" on the wheel to the skills you already have. This adjacency is determined by the game’s internal Skill.txt file and has been mapped by the community into the famous Heroes 5 skill wheel 16. heroes 5 skill wheel 16
Academy (Arcane Omniscience): The hero knows every spell in the game at the Expert level. Since patch 1
Players often aim for a core set of 8–10 essential skills, leaving room for situational picks. Example for a Magic hero: From there, you can only learn skills that
Patch 1.6 (and the standalone expansion Tribes of the East) was the turning point. The developers introduced the Skill Wheel, a diagram designed by the community (specifically the fan "Marcel") and officially integrated into the game manual/help system. It visually mapped out how skills connected to abilities and ultimate perks.
Here is the "long story" explaining the context, the mechanics, and the notorious difficulty of the Skill Wheel system.