[Taqwus]

Very, very nasty one, however -- since the population loss is near-total and takes place regardless of who wins the initial battle. There's no spell that can directly cause that, although there are ways to increase the odds of something bad happening IIRC (Baleful Star, IIRC; maybe Doom Horrors still have this property. Have not tested a 'fortune-teller' with a massively negative fortune-telling chance -- it'd hurt certain nations by neutralizing fortune-telling, if it worked).

It would not surprise me too much if this one were linked to scales -- ex. being more likely if you have magic, death and misfortune or some wacky combination like that, and extremely rare otherwise.