It is unknown what the Mercenary Lord's name and race really is, but many assume that Soulblighter payed him handsomely for the use of brigands and dark archers in his force. Some speculate that he was a long time friend of the Fallen Lords, others say that he was a friend of Soulblighter only--whatever the case, the Brigands and Dark Archers enjoyed gaining money and causing problems for the West. The Mercenary Lord was behind this all--a Kingpin of crime and wealth--his sole reason for existing was for the love of money and he personally hated Alric's nobility and goodness.
There is even some speculation that he is Finn, the son of Ravanna and Damas. If that is the case, then he could be far more powerful than just an average Mercenary Lord. The Mercenary Lord is also known as the Untamed King.
It is considered speculation, but if the Untamed King really is Finn, and it states that the mercenaries fought for more than just money, and that vampyres were to be implemented in the Myth world--then it should be noted that Finn was probably the first vampyre and the promise that the mercenaries got was eternal life with sentience. The promise of becoming an eternal vampyre probably appealed to many mercenaries--though this is a wild guess at best, this notion fills in the gaps left by the Myth story, as money for the mercenaries could only go so far as they knew eventually the world was going to be under the Dark. Thus, Finn being the first Vampyre and the Mercenary Lord solves the gap in the mercenary's logic and compliments the myth story as a whole.
If the following is used, then it is obvious how and why Phexios had such a difficult time in facing this opponent, though Phexios received scars from him, Phexios brought the Untamed King to his knees, and eventually Finn escaped. Finn probably had a close business relationship with the Baron. The enchantment that was broken by Cruniac was probably Finn's enchantment over the guard in the level "The Baron."
Finn as the first Vampyre in the Myth world, boasted many powers and abilities. He could change into bunches of bats and fly like Soulblighter did with his crows, Finn could hypnotize warriors into doing his bidding, he could change other warriors into vampyres, when causing wounds to his enemies--he could regenerate by getting their blood on him, Finn could turn into white mist and ambush opponents, and he could also teleport. Besides these powers, Finn was very fast like his father, he bore his mother's blades, and could attack like she did in Myth 3, and finally--Finn was considered as good with a Longbow as Oderic is.