[Taqwus]

It's a Conj 9 spell. They're supposed to be pretty darn powerful.
It's not a terribly strong army, however, and unlike Army of the Dead and similar spells, it only hurts one province per cast. If you can find weak provinces to knock over using remote summons, summons which let you keep the units are much more powerful because you can set taxes to 200%, up province defence if you like, and move the survivors to attack nearby weak provinces. In addition, you can't really combine Ghost Riders with any other simultaneous remote summons, e.g. Army of the Dead, because they're not friendly... but summons like Army of the Dead, Horde from Hell, Imprint Souls, Call of the Winds etc are *stackable*. You can safely mass-cast these other summons into one province if you like, trash the locals, and then disperse to cause more damage in subsequent turns if it seems appropriate, or keep 'em as a single deep-penetration force.