From LSWiki

This charm summons a terrifying necromantic spirit from the remains of a dana. These are considered followers, and will provide additional damage in combat. They can be kept, and use the "Soldiers" keep capacity. They accept commands given in anglic, and can fly.

Your familiar must be alive and present in your environment, along with the corpse of a dana, for this charm to work.

Description of the Banshee

This is a banshee with translucent gray skin, translucent raven-black hair, and translucent
green eyes.  This is a banshee, a ghost-like creature said to be spirit of a dana caught on the
way to, or conjured back from, the underworld.  Different legends have them arising in different
ways: as the result of an injustice, as an omen for an injustice, as a consequence of unrequited
love, and so on.  But all the legends agree that they are female in form, and typically quite
beautiful.  She has a webwork of sparkling light within and around her.  
She looks about eighteen and a half dimins tall, four and seventeen twentieths dimins wide
and one and a fifth dimins long.  

Usage

concentrate on conjuring a banshee