There are currently two major mercenary guilds in Northland.
The Velkross Guild, based in Keldoria, is widely agreed upon to have the strongest, most experienced and effective warriors. For those situations where you really need muscle and fearlessness, Velkross will have your men (or orcs). Velkross is known to be associated with Zhakhustra, and one can occasionally acquire undead mercenaries through the proper channels. Most of Vekross recruits are from Keldoria and the surrounding areas.
The Continental Guild, based in Vespice, tend to have both more expensive and more cowardly soldiers, but they also tend to be more civilized and have a reputation for loyalty and trained professionalism. If you want smart soldiers who are capable of using complex tactics and disciplined formations, Continental is what you want. They also offer airships and sea escort services. The Continental Guild recruits from places ranging from Dalstrom to the Dreaded Isles.