When cast, this spell had a chance of enticing a particularly intelligent and exceptionally strong ghoul to the service of the spellcaster. It wasn't always successful, but was more effective the more powerful the wizard was. Occasionally, the spell summoned forth a ghast, which typically attacked less-capable necromancers.
If the casting was a success, attract ghoul created an empathetic link between the ghoul and the wizard, which lasted for the range of one mile. The undead creature obeyed simple commands as issued by the wizard, so long as they provided it with a suitable lair and ample food. The ghoul would wander away if it wasn't well cared for.
The casting of this spell was a draining process, and could only be cast once per year. It required verbal and somatic components, as well as material components in the form of a rather expensive censer of myrrh and herbs.