If I managed to wipe out a rival group of wizards who had something as powerful as MR while I had none, the first thing I'd do is hunt for that secret. But then, I am a greedy PC. 
Exar's right. There would be several remaining Bjornaer and a small number of non-Bjornaer who had undertaken the Secret Name Initiation.
It makes a magus immune to sympathetic magic forever. Presumably, Symbolic magic, as a form of sympathetic magic, wouldn't work on them.
So, who's left? Well, most of Clan Ileftu with their Divination special ability, which will make finding whoever is responsible fairly easy.
Perhaps one or two from each of the other clans. Mostly those who have need of enhanced defenses: hunters and champions.
There may be five (max) who received this blessing who are members of other houses, generally those with a military bent.
Did I mention that Novogord is a favoured tribunal for Bjornaer? And a specialty of the House is tearing apart large numbers of enemies in a defined space?
The presense of a tiny remnant of the Order (these Bjornaer, a few immortal magi like the Criamon spectral primi, other living ghosts and, perhaps, the Mercere Redcaps if the Symbols speculated "wizards") would mean that there would be a whole other dimension to the predicament of the apprentices. They may or may not know of these potential survivors. They also may or may not know of places where they can find the great resources of the Order. There will be a race against time to get there.
And how will the surviving Bjornaer react to the apprentices when they find them? What are the former familiars up to? Are they interested in vengeance? Will they try to claim the other Mystery Domus Magae, including Verdi and the Cave of Twisting Shadows? What would apprentices of those Houses say? And what of the grogs and companions of the other covenants, especially Verdi? Will they be able to master the amazing tools of their former masters and allies?
The slight remnants of the Order make it so much more interesting...