Recruit Special Units

You can get rare troops from mercenary captain, special recruiters, dungeons or by capturing a city and constructing specific buildings.
The Mercenary captain in the tavern of towns will have special troops for hire apart from the crappy normal mercs and a very low chance for wizards (spiritmancer, dark wizard...). It depends on each region:
* Redstone valley: ember infantry. Very low chance.
* Crying rock: the northern-most town (amaranth or something) has special monsters like dryad, puppet, centaur... for hire; the other towns have Radiant troops for hire. All are at equal chance to crappy mercs.
* Mountain: alpine guerrilla troops. These are trash, don't bother.
* Desert: mastigure troops. Quite good
* Umbra cliff: wildfire clique troops. These are high quality elite troops, especially wildfire silver warriors. The chance increases with your level and they will replace crappy mercs as normal hire later on.
Special recruiters allow you to hire troops from their guild. These recruiters will move between towns and maps after 1-2 weeks. They all have predetermined names.
* Mastigure Explorer recruiters: Lut and Umah. Allows you to recruit Mastigure scavenger (specialized anti-monster and anti-wizard troop) and Mastigure ravager (okay-ish spear troop).
* Ember Guild recruiters: Rey and a girl I don't remember her name. Allows you to recruit Ember Infantry (superb bruiser/tank troop) and Ember engineer (siege machines on foot).
* White Rose Society recruiter: Chariva, he is in the city hall (with faction leaders) instead of tavern. Allows you to recruit White Rose Cleric (single healer) and White Rose Apprentice (AOE healer). Both out-heal Nasir healers by a large margin.
Dungeons allow you to hire beasts and monsters.
+ 4 dungeons scattered around umbra cliff and 1 dungeon in snow mountain, look like a prison gate with 2 black wolves in front. Allows you to recruit beasts and low level monsters. The best choices are Sandworm (evolves into a powerful unit later), Aborean, Centaur and Bahar.
+ 1 special dungeon northwest of Snowridge in snow mountain, looks like a teleport portal. Allows you to recruit high level monsters like dragon, phoenix...
Capturing a city is required to recruit Thur siege ram, Dhib yeti and any kind of Ifrit troop. Always try to capture an ifrit city as soon as their realm is open. These guys are superb.
You can also befriend roaming bandit factions to recruit Desert brotherhood bandits (cost-effective T4 infantry) and wildfire clique troops.
(From https://www.reddit.com/r/SandsOfSalzaar/comments/s8f3uy/rare_troops/)