Resisting Spells is a skill that determines your ability to resist magical energy. The skill is vital for most, if not all, PvP character builds, though many PvE character builds also focus on Resisting Spells as well.
Although the name suggests that this skill will help with resisting all spell damage, this is not the case. This skill helps lessen the impact and severity of various harmful spells, and decreases the duration that negative buff spells (such as Clumsy) take effect. Certain spells, such as Poison and Mana Vampire, can be completely resisted. Further, Resisting Spells lessens the impact of harmful spells, such as Explosion and Flamestrike; it does not completely resist damage.
A players chance to resist magical energy is calculated by comparing their Resisting Spells skill to the Magery of their attacker, as well as the level of difficulty of the spell he/she is casting (i.e. Spell Circle). This assures that it is always more difficult to resist a Flamestrike spell as opposed to resisting a Magic Arrow, as the Spell Circle is included in the Chance To Resist formula. Further, it also gives a clear advantage towards having higher levels of Magery, as even a Fireball can become extremely deadly when it is being thrown from a Grandmaster mage!
The calculation of chance to resist a spell is a bit complicated, but consider the following as a guide:
Take the following two values:
A. Victim's Resisting Spells / 5
B. Victim's Resisting Spells - ((Caster's Magery - 20) / 5) + (Spell Circle * 5)
Then the following calculation applies:
If A > B, then the resist chance is A / 2
If B > A, then the resist chance is B / 2
This is the base spell circle Resist bonus. There is also an armor bonus to resisting spells, as follows:
Armor Resist Bonus = 0.115 * (([Armor Penalty - 0.1 OR 0, whichever is greater]/1.25) Resisting Spells / 100)
The final resist chance is then calculated as:
Spell Circle Resist + Armor Resist
If the spell casted is not a direct damage spell then armor magic resist bonus will always be zero. In PvE, spell vulnerability also affects resist chance.