I think that pressing F should activate the rune which Bardin has but the exact rune that he has should be decided by his Talents. To this also comes that I feel that the rune should be a buff to existing weapons or something else rather than an attack. With at least three, or more, choices between which rune we want we can have more diversity in the career and its use and builds.
But I do very much like the idea of sapping magic. In my opinion one use of the ult could be a “Rune of Dispelling” which will cancel any spells by leeches, stormers, Rasknitt, Halescourge and Nurgloth and perhaps even have a chance to stagger them a bit. Or it could be something like “Rune of Clearence” which sends out a shoch wave which staggers nearby enemies. The last one might be better as it could mean it useful also against non-sorcerer foes.