As in simply general "I win and you lose" spells, or "bring unto the apocalypse" spells? For stuff that's a bit of both:
Burden of Time (Thaum 5; D5): Murder on the living, especially for nations with old-age mages or that lack Growth scales. Worse, the nations that will most often cast this (*cough* Ermor) will have massive Death paths on their pretenders, making dispels hard, and have other factors (*cough* Ermor) already making them a target.
Utterdark (Alt 9; D9): Permanent darkness everywhere, causing major havoc for anyone without darkvision or other units not impaired. Random shade attacks are just icing on the cake.
Astral Corruption (Blood 7; B6S6): Casting or forging now gives horror marks if you aren't using blood magic. Horror marks cannot be removed, and they stack - the more you have, the more your mage gets targeted for assassination by random horrors, and the stronger those horrors are. Given that the late game pretty much runs on magic, this is...entertaining.
More in the lines of the world of darkness, but less effective in a general practical sense could include Foul Air (all injuries cause disease), Illwinter (increases global cold; thematically the coming of Ragnarok), Perpetual Storm (unless you're a flier nation like Caelum, in which case it's a *huge* pain), and Wrath of God (roasts heretics, but tied to your dominion's extent). Maybe Enchanted Forests (the forests themselves awaken to sing your praises and sweep away opposing faiths) as well, but that's more of a Gaia's Revenge than a world of doom.
For stuff that's powerful, but tends to be a bit more questionable on how dangerous they are:
Well of Misery (Conj 8; D6): Everyone in the world gets healthy; yay. Unfortunately, the caster is turning all of that disease, aging, and decay into pure, concentrated Death gems. Most gem generators like Mother Oak probably qualify, but this one gets special note for actually giving other players a reason not to immediately try to purge it.
Gift of Health (Ench 7; N6): Everyone who worships you becomes healthier. This one's more likely to be poached because someone else wants the effect, or fears you'll be Tartarianing it up (does it still work on Tartarian insanity?).
Forge of the Ancients (Con 7; E5): I lied. This one will likely make you a target, because it means you're going to be mass-forging a whole bunch of nasty stuff.