[LDiCesare]

Assassin spells are anonymous, but often easily tracked back: 'who on this map could be casting 5 earth attacks against me?'.
Assassins are not very reliable but their main effect is to cause people to use bodyguards. This can be relatively useful if you can sneak an assassin near a research center where assassins will easily slay sages for instance. Equipping your assassins with a medallion of vengeance can also be quite efficient against human commanders without commanders, but is both expensive and random.

I did have some fun with a black-hearted vanjarl using Percival the pocket knight as a decoy against bodyguards and spells. Surprisingly effective, but still very sub-optimal.