Magic systems is the rules of the magical effects in the game. How it works depends on the setting, the gameplay and the need to balance the magic power against other elements in the game.
How you get access to new spells as the game progress.
Level: As you get better in your magick skill you get new spells.
Learn: You buy them, find them or train at a master.
Item: You need to acquire a item that allows you to cast the spell. For example a rune.
Schools of magic where there is a group of spells that is used by each school. It can be that one is limited to a single school or that spells from other schools are a bit more expensive to learn.
Alteration, Black Magic, Blood Magic, Conjuration, Destruction, Divination,Elemental, Illusion, Mentalism, Mysticism, Nature, Necromancy, Restoration, Summon, Transmution, White Magic
A spell consume physical objects like talismans or reagents when it is cast. If one does not have them the spell can not be cast.
The game can have a pre-defined list of spells or allow the player to create his own spells while playing.
If the wizard can throw a unlimited number of fireballs they would rule the world. To balance them against other character it is common to limit the number of spells they can can cast.
Magic Points: Each magic user has a mana pool that can be used to cast magic and each spell cost a certain number of mana to cast. Lost mana can be restored by sleeping, potions or slowly over time.
Skills: In a skill system spells are connected to skills. Each spell has a difficulty and when casting a spell one need to pass a die roll on the skill. Each time a skill is used a temporary difficulty boost is placed on the skill so using to same skill to often is impossible.
Spell-Slots: Each magic user has a number of spell slots that can be drained each day to cast a spell.
Ultima Underworld - 1992
There are magical runes to be found in the world. By using up to three of them in a certain order a spell is formed and can be cast.
Legend - 1992
Runes can be bought and combined to form spells. There are four director runes that choose in what way the spell is cast and 12 effector runes that choose what the spell do. Each rune used consume some ingredients so there is a cost on money to cast spells. There are some limits on how spells can be combined, mostly around the direction runes,
Ultima 7 - 1992
Buy spells for spell book. Casting a spell costs mana and spell regents.
Dungeon Siege - 2002
Find or buy spells. Two schools of spells and to cost a spell one needs to have a higher skill then the spell. Gains skill by casting spells of the school. Casting a spell consume mana.
Skyrim - 2011
Five magic skills, one for each school of magic and each spell belongs to a school. Casing a spell cost mana that depends on the skill. A spell can be learned if the skill level of the school is higher then the spells difficulty. Spells can be learned from tomes, purchased or found.
Ultima IV and Magic - 2010