[Taqwus]

Heh. Murdering Winter, Fires from Afar, Flames from the Sky, Leprosy, Beckoning... even Baleful Star (can curse units exposed to it) and Rain of Toads (can cause disease) can be used to harrass numbers. Then there's Mind Hunt (but don't use against astrals...), Earth Attack, Wind Ride, Seeking Arrow, Manifestation...

If you're parking lots of commanders in one spot, it'd be good to use a dome. Domes of Solid Air are nice in that they have a high success rate (80% block chance) and don't need extra gems to Last (but do require Air-5 and 20 air gems). Multiple air domes can stack in the same province, haven't tried mixing them yet.

Still, I think the most serious "Kill me now" spell has to be Utterdark. It causes 90% reduction in taxes AND resources, shades attack your enemies' provinces, precision / attack / defense all tend to suffer especially for non-undead. That's pretty much a universal declaration of war in the off chance that somebody pulls it off (not cheap; requires Alteration 9, Death 9, base cost of 100 death gems).

90% income reduction? Living troops probably deserting en masse? Inability to replace heavy troops in a reasonable amount of time? Ouch.