Mirror image is a powerful but unreliable defensive spell. It conjures images of the target that can each fully absorb one hit of damage or one offensive skill/spell. At its best, it can easily stop 1000 or more points of damage for about 100 spell points. At its worst, it will do nothing at all. Due to its limitations, many newbies never learn to use this spell correctly, but when used properly it is quite useful. Mirror image is probably most valuable for riftwalkers and for 3 man vertical parties.

Drawbacks: High powered monsters may ignore images, the spell’s duration is random and low, the number of images created are often very random, the spell’s cast time is quite high at 12 rounds. Area spells destroy all images. Doesn’t protect against stuns, for no justified reason.

The in game help reads:

No saving throw.
Casting time: 12 rounds.
Type of spell: neutral spell.
Affecting stats: int.
Cast type: protection.
Spell point cost: 93.
One family of magicians which have been around since the conception of magic,
is that of the illusionists. Using magic to fool the naked eye was one of the
first testing grounds of magical power. One of the most useful applications of
illusionary magic, is that of the mirror magic. By casting this spell,
duplicate images of the target will appear, confusing any attacker into
hitting false targets. Unfortunately, moving will cause the mirror images  to
disappear.