Quite by accident, I believe I have created a world-ending spell. I was thinking out loud, working through a chracter class that can summon permanent mundane weapons, armor, and normal humans (level 0 humans, no armor or weapons), use his mighty charisma to sway them to his side, then use his intrinsic training/leadership skills to train them into a fighting force. Realizing I may be able to summon humans with abilities, I worked through a spell... and realized I had made a terrible mistake.
Now, this may bend the rules a little, since it relies on summoning as an innate ability (spell-as-custom-power), but it stays within the rules as written, at least as far as I can tell. If not, a little tweaking and it will anyway. So, let's start with the worst part; the character summoned is of the following class:
Nuke Class (3 build points)
Fighter 2; Thief 1; 1 thief power: Backstab, 2 custom lvl 1 spells-as-power (1 round casting time, 1 per hour). 1200 XP to level 2 if you're making a player class.
Then, the spell itself (which is also the first spell-as-power):
Summons 4 level 1 creatures of Nuke class within 120'; last 1 turn.
Summon 4 creatures. Creature may have up to 1 HD. Creature may have up to 1 special ability (*). Summoning restricted to certain monsters (e.g. djinni). Targets summoned from thin air (alternative plane, etc.). 120'. 1 turn. No saving throw permitted. Arcane.
Math: (150)*0.15 *1 *0.7 *1 *1.25 *0.5 *1 *1 = 9.84 = lvl 1
And the second spell:
The caster can throw his voice to up to 100 creatures within 120' of himself; the effect lasts up to 1 turn.
Create illusion, non-visual mono-sensory. Illusion limited to 1 obviously fake, harmless category. Illusion dynamic (animated/moving). Illusion under control of caster for duration. 1-100 creatures within 120' radius. 0'. 1 turn. Beneficial effect. Arcane.
Math: (3)*0.2 *1 *2 *4.5 *0.5 *1.75 *1 *1 = 4.73 = lvl 1
To initiate the nuke, cast the spell and summon 4 copies. Tell them to summon as well, and to tell all their summoned creatures to summon also. After 1 round, there are 4 copies; after 2 rounds, there are 16, then 64, then 256, then 1024 after 5 rounds. 16 rounds in, the summoned creatures likely outnumber humanoids; after 1 turn (10 minutes, or as long as the original spell lasts), assuming there is still anyone to cast anything, there will be 1.329228 x 10^36 copies. Assuming an average weight of about 150 lbs, the copies weigh 45469601 times as much as the sun - enough to create a black hole, and wipe out not only the entire planet, but likely a significant portion of the solar system. While the creatures can be magically dismissed, teleported, warded, or otherwise removed, the sheer number of creatures will likely overwhelm any number of spellcasters.
But stepping back - the character could, instead, summon the 4 copies 120' in front of himself, then tell them to summon only one set of four 120' in front of them, and so on; additionally, the second round after being summoned, one from each set of summoned creatures should activate their communication spell. After 9 minutes, the second from each cluster of casters summons four more casters to keep their number up, thus leaving an uninterrupted chain. Since each summoned creature will only exist for 10 minutes, no single caster will be "awake" longer than that, which means the entire chain can be kept up as long as need be, 24 hours a day. If anything attacks a section of the chain, the third and fourth creatures merely use their summon spells, then charge into the fight; every round, the number of fighters is multiplied by 4, until the enemy is overwhelmed by sheer numbers of backstabbing fist-fighters. Even if they can only hit on a natural 20, they can still land thousands of blows in just a few rounds. Or, y'know, box the enemy in and drop summoned fighters on their head from 120' up.
At 120' per round, after 24 hours of travel, the original caster could send a message through his chain up to 196 miles away, as the crow flies. Which, as a note, the message could be "as soon as you get to Castle Gonnabiteit, summon a horde and destroy the castle. Then just stand around for 10 minutes until you vanish, leaving no trace."
But, maybe destroying the world isn't your thing. How about the entire omniverse? Use the following class instead:
Fighting 1; Thief 2 (5 powers): Backstab; spells-as-powers: Multiply and Telecom as above; Teleport (1 round casting, 1 per month), Adaptaion (1 round casting, 1 per month)
Then cast Adaptation, Teleport, Multiply. There is a small chance that the creature will be "lost" to some unknown plane, and can start creating copies there; otherwise, they teleport into space, and start replicating. Eventually, they fill all space between planets, and trigger a super-massive black hole; still, they can teleport away faster than the hole can draw them in, which means that soon, they fill the entire known universe. And the unknown universe, since they've "accidentally" teleported there, too.
Is my math wrong somewhere, or did I just end the world with a first level spell?