1: Take Locate City, a spell with a range of ten miles per level
2: Apply Snowcasting (Frostburn) to it, making it a [Cold] spell
3: Apply Flash Frost (PHBII) to it, making it deal 2 cold damage to everything in the area
3.5: Add Fell Drain (Libris Mortis) making it deal 1 negative level to everything that takes damage from it.
4:Apply Energy Substitution (CArc) to it, making it an electric spell
5: Apply Born of the Three Thunders to it, allowing a reflex save to avoid the damage and changing the damage type
6: Apply Explosive Spell to it, forcing a second Reflex save to avoid being blasted to the edge of the area and 1d6 per ten feet traveled
Ok, so you have everything within 200 miles being pushed to the edge. Everything that has one hitdice or less become Wights somewhere between 1d4 rounds or by the next day. Going by Leadership standards. There is 10 npcs with 1 HD for every one with 2. There are 2 NPCs with 2 HD for every NPC with 3. There are 2 NPCs with 3 HD for every NPC with 4. So on, so
Pretend there were 10 million NPCs within 200 miles. That is 8,333,335.982 level one NPCs who will become Wights. 83% of people do. The rest just die. Since they dont become Wights immediatly, the Wights do no die. The corpses, however, could be pretty mangled but RAW there is nothing against that.
(3.5 rules)
Thanks to Dictuum Mortum for the idea.
EDIT: I think you could apply widen spell to this, am I correct?