The Olden Era skill system is designed as a hybrid of the classic Heroes III system and the perk system from Heroes V. The developers emphasize that they are not trying to completely reinvent progression, but rather expanding upon a familiar foundation. Source: Devlog #11.
Gaining Experience and Levels
Heroes gain experience from battles, visiting certain objects on the map, and treasure chests. Upon leveling up, a hero can upgrade one of their already selected skills or choose a new one, and also receives a bonus to one of four characteristics: Attack, Defence, Spellpower, or Knowledge. Certain skills and artifacts increase experience from these sources, and some objects on the map can teach a new skill or improve an existing one.
Starting Skills and Limit
Every hero always starts with a unique skill of their Factions, similar to Necromancers in Heroes III or all heroes in Heroes V, and usually one additional skill. Starting skills cannot be changed; they depend on the hero's specialization. For example, a hero specializing in a school of magic always starts with the skill of that school. In total, a hero can learn 8 different skills, including starting ones.
Sub-skills
Each skill has 5 sub-skills. When upgrading a skill to advanced, the player chooses 1 of the first 3 sub-skills; when upgrading to expert, they choose 1 of the last 2. Sub-skills provide significant bonuses, can change the hero's role, and are often strengthened by the presence of another specific skill, creating soft synergies.
Subclasses
Subclasses are inspired by Heroes VII, but in Olden Era they are optional. Each hero class has 2 subclasses that require certain skills and sub-skills. During progression, the game notifies you if a selected skill or sub-skill brings the hero closer to a desired subclass. Subclasses reward developing a hero in a specific direction, but a strong hero can be built without them.
Balance and Variety
The goal of the system is to reduce the pressure of a single "meta" path, provide more development options, and make player decisions meaningful in every game. The developers note that sub-skill balance is still changing, which is why the full list of sub-skills has not been revealed in the log.