Galdred
Studio Draconis
If 4X are ok, it is hard to beat dominions indeed.What game (RPG presumably but it can be strategy with mages or something alike) has the most devastating, totally annihilating magic?
Dominions, and it's not up to discussions. Literally dozens of world-ending spells alone, from causing it to freeze in eternal winter or covering it in pitch darkness to tainting all magic with lovecraftian horrors and unleashing mad ghosts of past gods to hunt all that is living.
For RPGs, well, there is Exile 3 with Quickfire spell that eventually burns an entire level of the game killing everything in it, and Ultima used to have Armageddon spell that kills everybody in the game except the protagonist and Lord British.
You can have horrors invade the realm and murder anyone trying to cast (non-blood) magic, meteorites randomly crashing down every turn, or even have the sunlight blocked or something like that. A lot of the end game spells are armageddon grade ones.
Master of Magic could work too, but it doesn't have the same scale of apocalyptic spells.
One funny aspect of dominions is that the world population is usually much smaller at the end of a game than it was at the beginning.