Mercenaries
What is a merc?
They are NPC controlled party members that are available as four types: melee damage, tank, healer and damage caster; they take a share of experience just as players do and level up with the purchasing player.

The melee damage and damage caster are intended to contribute DPS while the tank and healer are to absorb or to heal damage respectively.

What are merc ranks?
Merc ranks determine how well equipped and likely to flee (if they believe the party will lose an encounter) from combat a mercenary is; initially apprentice and journeyman mercs are available, with apprentice mercs considerably cheaper than journeyman mercs.

Which expansion were they included in?
Seeds of Destruction.

How do I hire a merc?
There are liasons available in the Plane of Knowledge (use the Find button for locating a specific race liason) where you can purchase a merc.

How advanced is the AI?
Good enough for the intended role, for example, the tank merc will attempt to hold aggro (and unlike pets, can take it from players) and refrain from chasing after pulled monsters before they have arrived at camp.

Additionally, most mercs (depending on their rank) have multiple "stances" which can change their behaviour, for example: the healer and damage caster mercs can be instructed to conserve mana and the tank merc to assist a designated character in combat.

How much do mercs cost?
They have an initial purchase cost (which is about 350 platinum for a journeyman I mercenary at level 30 and 1000 at level 50) and a much smaller upkeep cost that is charged every 15 minutes. They can go as high as 10K for level 90 journeyman V, with a 95 upkeep every 15 min.

What merc types does each race have?
The available types for each race are listed below:

Dark Elves have melee damage, tank, healer and damage caster.

Drakkin have melee damage, tank, healer and damage caster.

Dwarves have melee damage, tank and healer, goblins for damage caster.

Erudites have damage caster and healer, kobolds for melee damage.

Frogloks (Guktan) have melee damage, tank, healer and damage caster.

Gnomes have melee damage, tank, healer and damage caster.

Half Elves have melee damage and tank, orcs for damage caster.

Halflings have melee damage, tank and healer, bixies for damage caster.

High Elves have damage caster and healer, brownies for melee damage.

Humans have melee damage, tank, healer and damage caster.

Iksars have tank, frogloks for melee damage and goblins for damage caster.

Ogres have tank, lizardmen for melee damage and damage caster.

Trolls have tank, orcs for melee damage and frogloks for damage caster.

Vah Shir have melee damage and tank, fungus men for damage caster.

Wood Elves have melee damage and tank, orcs for damage caster.

What mercs are available that are not a playable race?
From the racial liasons (also listed above): bixies (damage caster), brownies (melee damage), fungus men (damage caster), frogloks (melee damage and caster), goblins (damage caster), kobolds (melee damage), lizardmen (melee damage and damage caster) and orcs (damage caster).

There are also additional mercenaries available as special rewards (details not gathered yet).