Newbie Question about Desert Tombs

[tinkthank]

I just wanted to check out the units of Desert Tombs (never played them, really), and see that the game starts with 1 lvl-3 Unholy Priest, but I see no way to recruit further examples of these or other unholy fellows. What exactly does Desert Tombs offer then? How do I get these unholy chaps? (And what am I supposed to do with them? Dont unholy priests basically just cast spells which increase the robustness of undead?)

Thanks much