A remnant from the schism war, where they performed heroically, if secretly, these specialists are still trained today, should the house need them.
There were many variations on this theme, each assassin having his own style and favored method(s). Today, they are few and far between, as the need for them has greatly diminished, and they're deployed even more rarely.
Being a Mage Killer is a dangerous and risky career, as the more senior magi are often quite difficult to kill via non-magical means.
Still, a mundane may sometimes succeed where magic has failed. More importantly, as precious as they may be, in times of war, these courageous men and women are usually seen as more expendable than a maga, or at least able to save time and magical resources to the house. After all, they only cost silver.
Suitable Virtues: Puissant (Social Skill), Affinity with (Social Skill), Cautious with (Social Skill)
Suitable Flaws: Overconfident
Suitable Skills: Brawl, Charm, Guile
Mage Killers often use special equipment created by the house minor talents, such as the house's mundane Alchemists. Reagents, Enriched Items, Minor Magical Items, you name it, so long as the job is done.