Most games give the player a list of pre-existing spells to choose between. Others give the player the option to customize their spell selection, up to and including creating entirely new spells from scratch.
The more options such a system gives the player, the more open it is to Loophole Abuse. Such systems are most common in tabletop RPGs where a human Game Master is present to prevent players from creating Game Breakers. It occasionally pops up in single-player Wide-Open Sandbox RPGs as well.
- Mages Of Mystralia features this as a central mechanic. Throughout the game the player will collect runes that they can attach to their basic spells, modifying them in a wide variety of ways. Later the player even gains "trigger" runes, which allow them to make spells cast other spells. One promotional video by the game's developers shows off how to combine runes to make an automated rotating turret, in order to show off a little bit of the system's depth and versatility.
- Nox allows wizards and conjurers to create custom traps (stationary ones for the former, living summons for the latter) with up to three spell effects that they already know. Once a trap is triggered by a enemy's proximity, all three spells are unleashed at once. On the downside, creating a trap requires enough mana to power all three spells, which is expended at once, without a chance to regenerate—if you don't have that much, you can't create that trap.
- Tower Of The Archmage is a game where you select one "range" rune, and then multiple "effect" runes. Higly customizable, also have "artifacts" wich alter the spells in a big way.
- In Treasure of the Rudra, any combination of letters the player can think of can be turned into a mantra and used as a magic spell, though there are certain words, prefixes and suffixes that have set effects. For example, TOU is the mantra for a basic lightning spell, and prefixes/suffixes like KAA~ or ~NAS can be added to change its' damage:mana cost ratio... or if you prefer, words like Electrocute or Thunderstorm can be enscribed and will also work as lightning spells.
- Eternal Darkness has a Rune System. Essentially, all you need are three types of Runes (Spell, Target, Alignment) and you can mix and match them into whatever spell you want to create. Medallions called Circles of Power have a number of slots you can use to mix and match, from 3 to 5 to 7. If you have the right runes you can cast a spell even if it's not on your list, though there are only a certain number of valid combinations.
- While Dungeons & Dragons games, post 2nd edition at least, tend to have very thorough options in the core rules, the Dungeon Master's Guide carries extensive information on making new magic and items.
- The importance of spell research has varied over the years with each edition change. In 2nd Edition and earlier it was very important, as wizards didn't learn new spells for free as they leveled - they had to hope to find a scroll of the spell they wanted, plunder another wizard's library, or research. Research declined in importance starting with 3rd Edition as wizards now gained new spells for free as they leveled, and the sheer number of spells published for the game made it easier than ever to find what you wanted already out there rather than trying to make your own new spells.
- 3rd Edition's Unearthed Arcana includes rules for "incantations" - ritual magic that anyone could do with the proper skill checks - and how to build new ones from scratch, or at least from existing spells. The Epic Level Handbook had a similar idea for epic magic, though that still relied on being a strong-enough spellcaster to develop and use them at all.
- The first supplement of REIGN includes an exhaustive guide on how to create new magic spells and schools, and encourages players and Game Masters to make their own.
- Ars Magica may be the Trope Maker, or at least Trope Codifier, for freeform magic in games. Spells use pseudo-Latin "power words" to create spell effects on the fly.
- Mage: The Awakening and Mage: The Ascension: If you've got the power and the skill, you can do it.
- FUDGE, admittedly a tool-kit game, includes numerous complete magic systems, most of which are rather loose on their own.
- In the Buffy the Vampire Slayer roleplaying game, witches and other magic-users expressly create their own spells ahead of time by "researching" them, then building them from the ground up via a checklist.
- GURPS offers several varieties of magic that work this way, with differing rules about creating spells. Notable forms include Ritual Path Magic, which assigns various types of effects to Paths of magic each of which is bought as a skill, and Syntactic Magic, where Nouns and Verbs are magical skills, and spells are built out of combinations of them. The magic system in the second edition of the GURPS-based Discworld Roleplaying Game is similarly flexible, allowing players to cobble together magical effects as required rather than trying to give game mechanics for every bit of supernatural whimsy seen in the Discworld novels.
- Universal or superhero systems that come with their own design-your-own-powers rulesets that then apply to magic as well quite naturally include this. Examples are the Hero System, Mutants & Masterminds, and Big Eyes, Small Mouth.
- In War Dragons, each dragon has two or three spells that come standard. But, you can also craft extras of those spells in your Forge to attach to dragons that don't normally carry them. The only drawback is that standard-equipment spells can be used as often as you get enough rage in your meter to use them. Crafted spells can only be used once or twice during an attack.
- The Elder Scrolls
- The Elder Scrolls II: Daggerfall has an extensive spell crafting system, where you can combine up to three spell effects. It is only available to those in the Mages Guild and to high-ranking members of the Temple of Kynareth.
- The Elder Scrolls III: Morrowind: Custom spell creation is an option at a number of magical merchants. The player chooses a spell's range (self, touch, or ranged), area of effect (single-target or Splash Damage), duration, and effects, then the game automatically assigns a Magicka cost depending on how powerful the spell is, theoretically maintaining game balance. In practice, it is hilariously easy to design game-breakingly powerful spells by combining synergistic effects such as Weakness to Fire + Fire Damage, or assigning a one-second duration to spells that increase the Persuasion skill (which only applies in dialogue, which pauses the game clock). There are so many spell combinations that it very quickly reaches Exponential Potential.
- The Elder Scrolls IV: Oblivion grants the ability to create custom spells to players who progress to a certain point in the Mages' Guild quest line or purchase the Wizard's Tower Downloadable Content. Like Morrowind, the player chooses a spell's range, area of effect, duration, and effects. The game features fewer overall spell effects than did Morrowind, and also places more limits on the combinations, but there is still plenty of potential for game-breaking with certain combinations.
- Evil Islands: You must craft spell before you use them, using a keystone and then adding runes to increase the attributes (damage, range, duration...) of the base spell.
- Path of Exile allows you to customize spells using the gem and socket system. The base spells (and skills) remain the same, but you can modify many other variables. For instance, you can customize the Fireball spell so it splits and bounces to enemies, fire multiple projectiles at once, uses health instead of mana, casts twice in a row, have it fired by stationary totem, even all of the above at once.
- Two Worlds II: Spells must be crafted using specialized "cards" which determine the spell's effects. "Effect" cards determine the type of spell (air, fire, poison, stone, etc.), "Carrier" cards determine how the effect is performed through the spell (e.g. a Projectile Spell, an Area of Effect spell, or Summon Magic), and "Modifiers" alter other attributes, such as projectile speed and effect time.
- Tyranny has this sort of magic system. Spells are built from a combination of "sigils", starting with a core type — fire, ice, health, emotion, etc. — and then adding modifiers like targets, range, area, strength, duration, and so on.