We're dealing with a qareen. It's an Arabian monster that whispers insecurities and feeds off discord. [...] It feeds off the strife, becoming stronger and effecting more and more people until it materializes into our plane. Right now, it's out of phase, it's invisible and intangible.
In I Couldn't Have Done This Without You, the unnamed qareen climbs out from the newly created Malivore pit. The Necromancer greets him - calling him his new friend - and presents him with a better offer than peace. The Necromancer can offer him freedom from death and all he'll have to do is pay a visit to the Salvatore School and bring him back what he wants; the mora miserium.
Emotion Manipulation: A qareen possesses the ability to whispers insecurities to those around him in order to feed off of the discord and strife it causes to become stronger.
Differential Plane of Existence: Qareens are otherwise invisible and intangible on their plane of existence and can only materialize when they've grown stronger; i.e. by feeding on discord and negative emotions.
Myrrh: Qareens are weakened against myrrh, though to what extent remains unknown.
Sinbad's Scimitar: Qareens are weak against Sinbad's Scimitar, though to what extent remains unknown.
Fairies: Qareens are weak against fairies and specifically against the fairy dust they exude from their wings.
The unnamed qareen was released from Malivore, though the Necromancer offered him something more than peace. He could offer him freedom from death and all he would have to do is go to the Salvatore School and bring him back the mora miserium. The qareen ultimately failed in the Necromancer's task.
Unknown (Dissipated into smoke)
In Islam, the Qareen is a spiritual double, translated from Arabic meaning "constant companion", and is classified among the Jinn-type creatures, such as Ablah, a jinni.