Known as the Suel arcanamachs on Oerth, the arcanamachs of Toril can be associated with the elite warrior-mage traditions of Netheril, Imaskar, or even Jhaamdath or Mulhorand. These empires were known for the great and terrible power wielded by their archmages (technically, Jhaamdath was known for psionicists instead), and so elite schools of combat training were necessary to fight the dangerous magic of an archmage's rivals.
The arcanamach, then, is an elite guard serving archmage masters, serving the greatest wizards of their empire with both sword and spell. They are also assassins and spies trained to slay powerful wizards of rival noble families; they also defend the wizards of their own families. They perfect their art by training against other arcanamachs in hidden academies, in hundreds of secret duels involving both spells and steel.
Since the days of the empires of old, the tradition of the archamachs survive only in a rare set of magical tomes known as the Grimoire Arcanamacha. None of the old academies survive in the present era, but swordsmen of sufficient skill can learn the art of the arcanamach by studying from the Grimoire Arcanamacha, or by studying at the feet of a master who has studied from those books. The Grimoire teaches secrets of stealth, heightened alertness, mental endurance, and combative spellcasting.
While in ancient days the arcanamachs were elite guards or special agents for powerful wizards, modern arcanamachs have no particular allegiances. They may serve as spies and assassins for anyone. Some are dedicated to slaying mages, making their careers out of hunting spellcasters for the sake of ideology or profit.