Sial is an alchemical preparation that has the nefarious property of destroying one's will and memory. Those who take it (or are forced to take it) lose all volition and self-awareness for the duration of its effect. It needs to be administered regularly to maintain its effects.
Sial is a prohibited substance according to Aesedran law and the Siepta also have proscriptions against its manufacture. To be caught using it, making it or keeping someone under its influence are capital crimes in Aesedra.