Reaver's Call is a powerful spell employed by Vellinos, and then copied by a Council of Magic representative during the events leading up to the release of Planar Shifting. It should be noted that the spell is Reaver's Call, and not Reiver's Call, and has nothing to do with the race found near the Landing.
The spell allows the caster to summon the spirits of the dead, allowing them to speak with them as if they were alive. It should be noted that after completing the spell, all of the local spirits will come through, and not just the one in question. During both instances of the spell being cast, the caster had to subdue the multitude of rampant spirits that came through to speak with the spirit in question, that of the Nameless Necromancer.
Unlike similar spells that might be found within a clerical or otherwise spiritual circle, Reaver's Call allows the caster to not only summon a spirit, but also to exert some level of control over the spirits will. Alongside this, the caster has the ability to defend him or herself against any attempt at retribution that the spirit may attempt, and also can strand the spirit in the mortal plane for as long as need be, if not forever, if they so desire.
The spirit or spirits that are summoned by this spell may simply take the form of voices, but in both instances of it being cast, the spirit of the Nameless Necromancer took a ghostly form before the caster. From what his spirit said, being summoned in such a manner is somewhat disruptive of the peace of death.
The spell has a somewhat complicated preparation. It involves creating a complex summoning circle with a special chalk. The processes do not need to be exact, as there seems to be some leeway in how they are performed.
In the first instance of preparation, Vellinos drew the circle with a chalk he had prepared prior to his appearance, and then bolstered the strength of the spell with the blood of those present, as well as with his knife, an artifact of immense power. After this, the spell was complete and he proceeded to speak with the Nameless Necromancer.
In the second instance, the council representative required some components in addition to those he already had, which were foxglove, dragonstalk, stargazer lily and murkweed. It took a day to prepare the chalk after he received these additional components. While he did not have access to the knife, nor did he use the blood of others to strengthen the circle, he did call upon the blood of Auriia, one of those present at the first instance of the spell, to recreate the pattern so that he could copy it. After the chalk was complete, he proceeded with completing the spell successfully.