TomeNET

(7.8d) Spell failure rate                                                       
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The chance to not successfully cast a spell, aka the spell fail rate is shown
in percent when browsing a spell scroll/crystal.
For mimic spells, the failure rate is shown next to their spell name in the
mimic power menu (accessible with 'm').

The fail rate of a spell depends on..

Magic schools:
    Nearly all magic spells depend on the spell level and your intelligence,
    some spells however use wisdom instead of intelligence, those spells are
    designated in (7.8) in their spell description.
    Currently, the only magic spell using wisdom is 'Genocide'.
Prayer schools:
    All holy spells depend on the spell level and your wisdom.
Occult schools:
    Some shadow and most spirit and hereticism spells use wisdom, the others
    intelligence. All unlife spells use wisdom.
Druidic schools:
    All druidic spells depend on the spell level and your wisdom.
Mindcraft schools:
    All mind spells depend on the spell level and your intelligence,
    except for 'Charm' which uses charisma.
Astral school:
    Most astral spells depend on the spell level and intelligence, some on the
    spell level and wisdom, as designated in (7.8) in the spell descriptions.

Runecraft:
    Runespells use dexterity, intelligence and spell level to determine failure
    rate.

Mimic powers:
    All mimic powers use intelligence to determine fail rate.

In general, when you learn a new spell that starts at a high fail rate, you
will observe a great improvement just by further training the spell's school to
increase the spell's level. Increasing your attributes (depending on what this
particular spell uses, see list above) will also help.
At a certain point however, usually when the spell's fail chance is already
rather low, the spell fail rate might not go down further from increasing its
level. At that point the only thing you can do to further reduce the spell fail
rate is to increase the stat that the spell uses.

A spell failure rate of 0% means that you will always successfully cast the
spell. However, certain status effects can increase your failure rate
temporarily or even completely prevent spell-casting, even if your fail rate
used to be 0%:

-Blindness
 Some spells can be cast while blind (will say so in the spell description)
-Confusion
 Some spells can be cast while confused (will say so in the spell description)
-Stun
 Stun will increase your fail chance by 15%, heavy stun even by 25%.
 When you're knocked out you can of course not cast any spells (or do anything
 at all).