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Keywords are:  'thief skill evasion evade'

Skill: evasion                  Lvl 37 Thie  (28%)
                                (important)   Requires: contortionism
                                Group: Thief defensive fighting skills
                                
                                Speed: average

Evasion is the thief skill used to dodge and deflect the full force of
incoming magical projectiles, such as fireballs and icebolts.

Magical spells are difficult to detect and evade.  Thieves not only
learn to twist and contort their body in order to evade the actual
projectiles, they also learn to track their opponent's movements and
anticipate the casting of the spells that generate such projectiles.

A higher dexterity can help with the evasion skill.

Certain kinds of spells cannot be evaded, no matter how skilled a
thief might be.  Spells like 'firefield' and 'tempest' affect
everyone in the area, regardless of their position.  Cleric spells
like 'harm' and 'flamestrike' are divinely guided to their target.
Furthermore, spells like 'magic missile' and 'demonfire'
include magical tracking as part of their casting.