The ability to make custom spells is one of the most powerful perks acquired by joining the Mages Guild. You can combine multiple effects into a single, powerful spell, and you can optimize the strength of spells to minimize the Magicka cost. There are even versions of some spells that can only be accessed through the Altar of Spellmaking.
A list of suggestions for custom spells can be found at Useful Spells.
 Making Custom Spells
Custom spells can only be made at an Altar of Spellmaking. There are two in the Praxographical Center at the Arcane University. These are only available to members of the Mages Guild who have completed all the recommendation quests in order to gain access to the University. A third altar is available within the Arch-Mage's personal quarters in the Arcane University once you complete the Mages Guild questline and become Arch-Mage. A fourth altar is found in Frostcrag Spire if you have the necessary plugin, but you must purchase the magetallow candles and upgrade the altar before it can be used.
When you activate the Altar of Spellmaking, the spellmaking window appears. On the left you are presented with a list of all the magical effects you can use to make a new spell. This list only contains the effects of spells that you know and are able to cast (see Acquiring Spell Effects for details).
Select a spell effect, and a new screen pops up where you can set the range (on Self, on Touch, or on Target), area of effect (if appropriate), magnitude, and duration. These parameters determine three things: the Magicka cost of casting the spell, the minimum level of skill required in order to cast the spell, and the gold cost of making it. In the case of Fortify Attribute, Fortify Skill and similar effects, this screen also allows you to click on the attribute or skill shown in order to choose the desired attribute/skill.
Once you are finished specifying the details, return to the main spellmaking window. You can then add a second (and third and fourth) spell effect if you wish. For multiple-effect spells, the overall spell school is the school of the most powerful single effect (using the base magicka cost of the effect (see equation below).
Name the spell and click Done. If your skill level is not high enough to cast your custom spell, you will be given an error message and returned to the spellmaking window. You will be allowed to create a spell that costs more than your total available Magicka. Once done, the new spell will be added to your repertoire.
There is a bug when creating multi-effect spells that may allow you to create a spell even though your skill level is not high enough to cast it. In determining whether to allow the spell to be created, the game checks the magicka cost of the single most expensive effect, instead of summing all the magicka costs as it should. Therefore it is prudent to pay attention to the message at the bottom of the spellmaking window that tells you what skill is needed to cast the spell, as the message provides the correct information. There is generally no advantage to creating a spell that you cannot cast: it would cost less gold to create that same spell if you waited until your character's skills were high enough to cast it.
An exception is in the case of Fortify Skill effects. Fortifying one's Luck or the relevant skill will reduce the cost of creating the spell. This does not allow you to cast the spell yet - even if you reapply the fortification - but does let you have access to the spell the moment that you do qualify.
 Spell Cost
The magicka cost of a spell is given by:
B = Base Cost / 10 M = Magnitude ^ 1.28 D = Duration A = Area × 0.15 Total cost = B × M × D × A
If M, D, or A is less than 1, a value of 1 should be used. The Magicka cost is further multiplied by 1.5 if the spell is a targeted spell. Base Cost is as defined in the construction set (see the Spell Effects page). A spreadsheet to calculate the cost is also available from Ong elvin's userpage.
The cost is further adjusted on an individual basis by a multiplier given below:
1.4 - 0.012 × Skill
Note that the Skill in this calculation is modified first by the player's Luck, and as usual the modified Skill is constrained between 0 and 100.
The gold cost is simply three times the Magicka it costs your character to cast the spell.
So, for example, 3 points of Fire Damage for 12 seconds will cost 36.7 Magicka (Fire Damage has a Base Cost of 7.5) and will deal 36 damage to the target. In comparison, a Fire Damage effect for 6 points and 6 seconds will cost 44.6 Magicka for the same damage. The lesson here is that spells are more efficient when they work over longer durations. Of course, this does make some sense, as the 36 damage would be dealt twice as fast in the second spell.
As a consequence, the Temple-provided Convalescence spells have two qualities that make them expensive and unnecessary for their purpose of healing allies: they are Targeted spells and have large Magnitude with no Duration. A more efficient custom spell can be created that is on Touch and with a longer Duration relative to its Magnitude. This is particularly useful for healing your horse.
Note that purchased/given spells sometimes have a different Magicka cost than their custom counterparts. Finger of the Mountain is always overpriced and Wizard's Fury is cheaper than if made through spellmaking.
The skill level requirement is determined from the base Magicka cost of the overall spell (summing the costs of all individual effects, and without adjusting based on your current skill level):
- Magicka < 26: no skill level requirement
- 26 ≤ Magicka < 63: requires skill of 25
- 63 ≤ Magicka < 150: requires skill of 50
- 150 ≤ Magicka < 400: requires skill of 75
- Magicka ≥ 400: requires skill of 100
For multi-effect spells, the requirement will be in the school of the single effect with the highest base Magicka cost. (Therefore in a spell with four Alteration effects and one Illusion effect, Illusion could easily be the school of the spell). Also, the experience you gain for casting this spell will only be in the primary school.
In multi-effect spells, the skill adjustment is applied to each component effect separately. Therefore, if your skill level is 100 in Alteration but 25 in Illusion, the Alteration component of a spell will be relatively cheap, but the Illusion component expensive.
If your skill levels are very different in two different schools, multi-effect spells can be used to create spells that you would otherwise be unable to cast. For example, if you are a Master of Illusion but a Novice in Destruction, normally you would be unable to create a Fire Damage 16 pts on touch spell (base Magicka cost 26). However, you would be able to create a spell with a combined Paralyze 6 pts on touch (base Magicka cost 285) and Fire Damage 100 pts on touch (base Magicka cost 272), because the combined spell would be identified as an Illusion spell. Actually being able to cast this spell might still be difficult, however, because of the high Magicka cost: even at a skill level of 25 in Destruction, the actual Magicka cost would be 356 (285×0.2=57 for the paralyze component, 272×1.1=299 for the fire damage component).
 Acquiring Spell Effects
You can only create custom spells using effects of spells that you know and are able to cast. For example, if your only Restore Health spell is too powerful for you to cast (e.g. it requires Journeyman level in Restoration, and your level is below that), then you cannot choose Restore Health as an effect.
An important point is that the various attribute-specific spells are considered to all be a single effect. Therefore, learning Fortify Endurance gives access to all seven Fortify Attribute spells at the Spellmaking altar. Similarly, the skills are also grouped. So a single Drain Marksman spell gives access to all twenty-one Drain Skill spells.
- The easiest method of acquiring spell effects is to purchase spells from merchants, mages guild members, and chapel healers.
- Spell effects can also be acquired from your greater and lesser powers (due to race, birthsigns or Doomstones).
Details on acquiring difficult spell effects are provided on the pages describing each spell effect. Two particularly difficult effects to acquire are Fortify Skill and Drain Attribute.
The Fortify Skill effect cannot be bought anywhere in Cyrodiil. However there are many other ways to get access to it. You can make all Fortify Skill spells if you have any Fortify Skill. The Fortify Skill page provides details on several methods that can be used to obtain this effect. (Note that Fortify Attribute spells are already readily available for sale throughout Cyrodiil from many spell vendors. Only Fortify Skill spells are unavailable for sale.)
The Drain Attribute effect cannot be bought from any spell vendor, and there are only four known ways to acquire this effect, as detailed on the Drain Attribute page. Acquiring Drain Attribute makes some powerful spells available at relatively low costs, since Drain Attribute is much cheaper than Damage Attribute if only a short duration effect is needed. Note, however, although Drain Endurance can be devastating when cast on the player, it has no effect on the health of any NPCs.
 Effective Custom Spells
When creating your own custom spells, there are a few considerations that can help to make your spells more effective.
- Weakness does not influence any Damage effects that are included in the same spell. In other words, you must cast a Weakness spell first, then follow it up with a Damage spell to get the increased damage due to the Weakness effect.
- Weakness can still raise Magicka efficiency. A 28 pts Frost Damage on touch spell costs 54 Magicka. However, a 100 pts Weakness to Frost (5 secs) effect followed by a 20 pts Frost Damage on touch delivers 40 pts of Frost Damage (a 50% increase) with the same base cost of 54 Magicka.
- When casting a spell which includes a Weakness to Magic effect on a target already affected by Weakness to Magic, the original Weakness to Magic effect will affect all effects in the spell up to and including the Weakness to Magic effect. Effects listed after the Weakness to Magic effect will not be altered, therefore it is generally best to make sure that the Weakness to Magic effect is listed last in any spells you create.
- If combining Soul Trap and Damage in the same spell, the Soul Trap effect must be listed first.
- You cannot include more than one of the same effect in a single spell. For effects which manipulate skills or attributes, the skill or attribute in question alone doesn't differentiate the effect. For example, you can't make a custom spell with both Fortify Athletics and Fortify Acrobatics effects, because they are both Fortify Skill effects. However, you could combine Fortify Athletics with Fortify Speed, because the latter is a Fortify Attribute effect. You could also create a Fortify Athletics and Absorb Athletics spell, because Fortify and Absorb are different effects.
- Due to the magnitude factor being raised to a power, a spell with a long duration is cheaper than a short-duration with the same total magnitude. For example, 10pts Fire Damage for 6 seconds is cheaper than 60 pts Fire Damage for 1 second.
- One implication of this fact is that combining multiple similar effects may result in a cheaper spell than a spell with just one strong effect. As an example, 20pts Fire Damage + 20 pts Frost Damage + 20 pts Shock Damage is a cheaper spell than 60 pts Fire Damage, while doing the same amount of damage to a target with no elemental affinities.
- Spells with different names can be simultaneously active, even if the spells do the exact same thing. This is a way to get around the 100 point maximum magnitude for spell effects.
- When a window is opened, or a conversation is started, any spell effects remain active indefinitely until the window is closed, or the conversation ends. In this way you can use "Fortify Sneak 100 pts for 1 second" to pickpocket almost anything, "Charm 100 pts for 1 second" to maximise Disposition, etc. A spell with a 100 pt Charm element on touch plus a 100 pt Fortify Mercantile bonus on the player with a duration of 1 second is an apprentice level restoration spell and will afford the player excellent prices from almost any merchant.
- It can be particularly effective to combine some effects with Invisibility. For example, a Conjure plus Invisible spell will allow your conjured creature to deal damage while you remain safely hidden. Another tactic is to combine an Invisibility effect with an area Frenzy effect to incite a riot among your attackers leaving you untouched.
- Although it may be tempting to create the most powerful spell possible, it is a much better idea to create ones with lesser costs. This way, you can cast it much more frequently, you are less likely to 'overkill' a target and it allows you to split the damage among several enemies should the situation allow. With a high Willpower, Novice level spells can almost be cast continuously.