The most important thing about the warlords - they come in certain dealing always! So, you can count on them :)
Each warlord has three skills, but you need elorites to unlock them. But even with all skills unlocked a warlord still has vulnerabilities.
The warlords have a race preference: a paladin will not guide the undead army, - but some of them can be combined. For instance dark elf Shikaree with succubus Ardath.
A warlord cannot be sold, sacrificed or summoned. You can only hire or chase them. You will need to pay him everytime you hire him.