Spells are the main way that the player has to deal Magic Damage and Heal. A big part of spells, especially those of characters that are meant to play the Support role will apply status effects and buffs or debuffs.
The big drawback of spells is that they cost mana, so if a player wants a character to be able to cast a lot of spells they should invest in increasing the Intellect of that character (Intellect increases the mana pool).
After using a Spell, a character cant do anything else that turn.
The player will have the option to permanently upgrade spells during the game, changing that way what a spell can do.
When designing spells I want to keep in mind versatility as I believe that if a spell can be used in multiple ways it aids in creating more interesting gameplay.
For example lets say that there is the following spell:
Petrify: Turns an enemy to stone, preventing it from moving and increasing its Armour by 10.
By simply changing that spell to be able to affect friendly units too, the player now has the option to use it to prevent an enemy from moving or to use it as defensive tool to prevent a friendly character from taking physical damage.
This is the type of design philosophy that I want to keep in mind when creating spells for the characters.