Illusion can be set by selecting the action on an ally's action box in the Ally Team tab. 135 points is awarded to the caster. The caster him/herself is not available for the copying, before or after a copy is made.
Illusion assumes some of the appearance of the target ally. Only level and class selected will be copied by the caster. Buffs, Debuffs, Health and Activity Points are not copied however. The caster's attack/defense stats, equipped gear and equipped General are retained.
Allies can see through the illusion, denoted by the illusion icon, but enemies and spectators cannot.
The illusion is still retained even if the power is unequipped.
Jaelle reduces the chance that the disguise is dispelled upon being defeated by 20%. Resisting dispel even if the opponent won should get you one or two more scouts checking out what's up if the enemy is not coordinating well.
Xelia is a "safer" but weaker version of Jaelle. Her additional resist rate is only 10% but is a guaranteed investment since she is a monthly.
Although Mirror Image has a different name and picture, it is considered to be the same ability, as its effect is the same. It cannot be used together with Illusion.
Illusion is best used in numbers. One or two mages assuming the appearance of one ally can be easily detected but several mages doing it can really make inattentive enemies to think first before doing any action.
Since activity points are not copied, it's ideal to copy an ally who is also active in the battle. Try to not leave your activity points end in 5 or copy an ally who also has their activity points end in 5. Otherwise, it will be easy for the enemy to see who is the illusionist.
Refrain from putting up Mana Shield before casting Illusion if the target ally is not also a mage. Buffs and debuff are retained and not copied so it will be a dead giveaway who the illusionist is if they have the shield up while their class is not mage. Mana Shield must also not be put up after casting Illusion as that would break the spell. If planning to have both spells active, copy an ally who is also a mage.
Try to refrain from casting buffs like Heal, Revive, Fortitude or Sentinel while the Illusion is up. This will count as a defeat to the disguised recipient and thus would break the spell.
It is not advised to avail of this power unless more in your guild can use it (that is, if you are playing enough mages). It could create confusion among the enemy team, forcing them to take time to regroup if they see a lot of copies. It can waste some of their time if they are not paying attention or coordinating well but their token use will be unaffected if they step back and think. Also, once the disguise is found out, they will be sure to mark the enemy formation and the mage illusionists so it'll be rare that they'll fall for the same trick twice.
It could add a bit of entertainment to the fight if the several mages do masquerade as someone even once.
Currently (almost) useless in 10v10 as the battle log blatantly shows who was imitating who.