The ability to create custom spells is one of the most powerful perks acquired by joining the Mages Guild. However, it is also available to those who have the Frostcrag Spire plug-in, after purchasing the Magetallow Candles altar upgrade.
By using the Altar of Spellmaking, one is able to combine learned magical effects into custom spells, choosing range (on Self, on Touch, or on Target), area of effect (if appropriate), magnitude, and duration. These determine what is needed to create and use the spell: the minimum skill level required to cast, the spell's on-use Magicka cost, and the price in gold to create it.
The spell cost is determined by the equation below:
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.
The cost is further adjusted on an individual basis by a multiplier given below:
1.4 - 0.012 × Skill
Useful tips to keep in mindEdit
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 100% 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 exactly the 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 maximize disposition, etc.
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.