Ikiryo were hostile towards a single living humanoid individual. When created, it traveled any distance to its target, even if they were on the far side of Toril. If the target was on another plane, the ikiryo was unable to find them. But once an ikiryo found its target, it followed them everywhere, even across planes. It followed the target until either one was destroyed.
An ikiryo drained its victim. The targeted individual became disturbed, fatigued, lethargic and restless. It also gradually lowered the intelligence of the victim, which meant if they were a spellcaster, they eventually lost the ability to cast or memorize spells.
Since it was a manifestation of psychic energy, an ikiryo was undetectable by the targeted individual except through certain spells. Detect evil or detect harmony spells were able to reveal that something was wrong with the individual, while true sight was the only way actually see the evil spirit.
Once detected, the targeted individual could use several spells against the ikiryo. Invisibility to spirits prevented the spirit from tracking the individual, but only as long as the spell was in effect. Protection from evil, 10' radius gave the individual a temporary respite from the draining effects of the ikiryo. If summoned via a summon spirit spell, the ikiryo's summoned form could be destroyed, thus freeing the targeted individual. An exorcism spell banished the ikiryo permanently.
The targeted individual could also find the person who created the evil spirit. If confronted about the presence of the ikiryo, the spirit vanished.
Ikiryo did not live in lairs nor collected any treasure.
Ikiryo were created as a psychic manifestation of a person's dark emotions of hatred or jealousy towards another, who became the targeted individual of the spirit. Only humanoids could create an ikiryo and they could not be created intentionally. Often the person who created an ikiryo was not even aware of its existence. Once created, the ikiryo could not be commanded or controlled by the individual who created it.