Light or Dark magic.

Well, the "Unholy Word" spell is definitely out of the question. It will be used against your own units.

Incorrect statement :)
The "Unholy Word" spell will not always be used against your own units.
I have successfully used a combination of Vampirism and, subsequently, the "Unholy Word" spell in practice.
The "Unholy Word" spell no longer affects your own units, and your units also regain health through Vampirism.
So, Darkness is a good thing for the Knight.

There is also a very good skill in Darkness that weakens attacks, specifically for the Knight, in combination with Training and Leadership.
If the Knight also has Combat Accuracy against a powerful race, it will be devastating.

There is also such a unit: Battle Griffin.
If it has Vampirism, it is almost impossible to kill it with brute force, especially considering that the enemy's ranged units will run out of ammunition first, and we can also put Leadership and the attack-weakening skill on this Griffin.

Once, I remember, in a battle against neutral melee units, I saw in the battle log that my Vampirism-enhanced Griffins dealt over a million damage :)
And this was in a real game :)