I do see a lot of spells and abilities used in battle,
but not all of them are avaiable to use.

Also it isn't really clear what the (side) effects are of spells.
Like stunning the enemy, shielding yourself, etc.

So I came to the idea, what about spell tomes?
Each hero does have a several amount of spell tome slots.
So your (main) character can learn other abilities,
even switch from them before a fight.

The tomes are like the plans for an purple item:
you need to find them, but also have requirements to
teach them to a hero.

For example: you need ... Intelligence to learn 'horns from hell'
or you need ... Strength to learn 'dervish attack',
or you need ... Command to learn 'piercing thrust'.

Also all abilities and spells need to be more clear:
what do they do? what are their side effects?
Just a short summary for each spell/ability.

About the special legendary heroes, they already have
a special spell from the beginning, so they keep the same in value
as before. They also have an improved command/strength/intelligence value.
The spell tomes wont influence the buyouts of the epic/legendary heroes.