RaisingK A high enough Mana stat is necessary to defend against them. At this rank, anyone below B rank will be instantly petrified, and even those who might resist it will be subject to reduced stats by one rank.
It's an Astral Lifeform thing. The Servants aren't technically alive, therefore it won't work on them.
Comartemis said: Something that always bothered me about Lelouch's geass... why didn't he ever order someone to "Obey my every command without question"?
Because he has a principle against robbing someone of their free will. He said something about that when C.C. suggested he geass Suzaku. That he finally does start doing it late in R2 speaks of how far he's fallen.