Light or Dark magic.

A mage's army controlled by a knight, advantages:
The mage has defense, attack, and leadership with a 2% drop rate, while the knight has 15%, 10%, and 15%.
Very good bonuses like ballistics, archery, resilience, and empathy.
The knight usually gets light magic, which the mage always has in the guild.
Stats go into attack and defense, which is very important for light factions.
The artifact shop can be used to supplement missing skills (although maybe level up twilight?) and knowledge, or to enhance attack potential.
Disadvantages:
Lack of mini-artifacts.
Gameplay focuses on the late game.
Chaos and summoning are not useful.
Hmm, and I think that's all.