Heroes 5.5 WarGame Edition - Tactical Edition.

Also, any grouping has its drawbacks, as in this case, the same types of units (Light+Dark) and magic users (Chaos+Summon) end up in a less advantageous position; the Orc can counter their schools guaranteed, with one skill, not two.

Well, as an alternative, you could make Light+Chaos... Dark+Summon... something like that. What bothers me more is that 2 branches will be compressed into one, and there will be a lot of skills.
And again, which branch should be the main one. Because something will have to be made as the first skill, and the rest as special skills.