[Edi]

Some mages appear as mercenaries and some you can summon. Mage being here anything from an actual mage to a demon or other fantastic being capable of casting rituals.

Some wizards can be recruited, but they do not know any rituals, just combat magic. There are no sites that allow you to recruit special units, but some sites increases the chances of getting offers from wizards, and other sites increase the chance of getting priest offers for recruitment.

And mages are always announced special recruitment. Something like Dai Bakemono for the Bakemono or goblin archers for Troll King can appear unannounced and disappear next turn, but wizard recruitment is always accompanied by a recruitment offer message.