I like the concept of an assassin or a rogue like mage itself, but not don't really see the character being a ninja.  I could also see stealth being an easy to abuse mechanic unless its implemented very carefully.  Even the Invisible Stalker if used very strategically could wreak havoc because while he's invisible he cannot be targeted at all.  A mage with a similar talent would be problematic unless it were borderline prohibitively expensive to cast or was like the voltaic shield for the wizard, where you pay a small amount at the start of your turn to activate a dodge or parry marker that reduced damage of an attack.  I like the idea of the corrupter or even opposing talent mages, light/dark, fire/water, earth/air, arcane/war where you pick a focus like the wizard but have the opposite cost 3x as much.