Gost riders

[sushiboat]

If you want Longdead Horsemen to keep, cast Pale Riders (Enchantment 5).

[Endoperez]

And I think there is a spell for sending undead to a province so that they stay in there. Something better than the Ghoul-making Arise Hunger (?). Enchantment 8 or 9, or figment of my imagination.

[Cainehill]

Yep. But unlike Ghost Riders, that spell won't destroy armies - it's more along the lines of Call of the Wild / Call of the Winds, useful for taking weakly defended provinces.

[Agrajag]

Army of Dead or Army of The Dead, an Enchantment spell.
Its a fun spell if you need to get some utility forces to a faraway province (such as pillaging a newly acquired province without having to keep its conquerors in the province).