I was going to make a spell called Sip of Cyanide, where by the caster makes a small portion of another characters drinking water turn into a poison.
Now I am assuming the person has the virtues to allow silent casting, or this would be a completely obvious spell.
Sound like a good idea?
You can actually add levels to the spell while making it to allow for special effects, such as a spell designed to be cast silently and withhout gestures. The other option is that it it's a low level spell but intended to be mastered with the effects of not having words or gestures
Cyanide might be an anachronism, anyone know?
Sip of Monkshood/Arsenic might work
As for how secretive it would be, the sigil of the caster plays a big role here.
Off the magi i play for instance, one has shadow and darkness as his sigil, so i might say that this spell cats by him would turn the drink dark and cloudy for a couple of seconds before returning to its normal colour, not so bad if its red wine in a goblet, but water in any kind of clear container might be noticed. Another magus i play has Runes and writing as his sigil. Not sure how that'd affect the drink, but if the drinker died maybe he'd get some raised welts in the shape of relevant letters.
I remember reading somewhere about a magus who had loud thunderclaps as his sigil, it'd be hard for him to something surreptitious like this.
Aside from sigil, if you cast without gestures or words then your spell will almost certainly go unnoticed.
Whatever this spell level turns out to be, you might want to make sure that your devious evil magus can cast this at a significantly higher level if he's going to be doing this to other magi. Magic poison still has to surpass the victim's penetration level.
Cyanide exists naturally in apple seeds and almonds but not in high doses. It is possible that your magus might know about it if he was as herbalist but if he did, he would also know that cyanide has a distinct odor and flavor that could tip off his victim. Arsenic wasnt discovered until the middle of the 13th century, still a possible anachronism. Monkshood or wolfsbane would be within the right time frame and so would my favorite, belladonna.
As gribble pointed out no virtues are necessary. Any magus can cast a spell totally silently if they take a -10 on their casting total.
Virtues or spell mastery just reduce this penalty.
Apparently only about 2/3 of people can detect the smell/taste of Cyanide for genetic reasons
Now that's an informed consent form I'd like to see...
It used to be 50/50, but quite a few of those that couldn't were poisoned. 
Xavi