You need to cast the 2nd level (Exceptional?) Necromancy spell called something like 'Army of the Dead' or 'Raise Undead Army' etc. This replaces your squishy flesh and blood human army with an undead one. You can choose between varieties of skeletons or zombies as you tech up. It comes with substantial benefits and the major downside that your armies move at half speed. Luckily you can field lots of armies cheaply. You can either play more defensively and use forts to catch the enemy for you or more offensively by forcing more mobile enemies to battle by herding them with multiple armies.

To do this, you need to open the Decisions menu, enact the 'Cast Realm Magic' decision and work through the decisions until you cast the aforementioned Necromancy spell. I am not sure if it has any costs to cast.

Oh wow thank you so much. I completely forgot about the other spells. Cheers!