So I'm playing a little bit of Ogre Battle again, and it occurs to me: there's Pumpkinhead creature in most Ogre Battle games, and this creature's attack is unique among the game's creatures: it always reduces its target to half its former hit points (unless the target is already a.) at 1 hp, in which case it either kills him or has no effect (I'm not sure which and/or depending on the game); or b.) undead, in those games where undead have no hit point score, in which case the pumpkin smash is one of three ways to kill it (the other two being clerics healing it and hitting it with holy weapons)).
And so it occurs to me, what if we incorporated this mechanic into D&D? My first inclination was to create it as a spell, but the downside to that is the players might get access to it. The other option is to give it to some creature as a special ability, in which case the DM (in this case, me) has more control over when the power gets used, and moreover can tweak it on the fly. On the other hand, I'm more likely to get feedback on it if it's a spell, and feedback is the lifeblood of this little test kitchen.
I'm sure somewhere in one of the bazillions of splatbooks, somebody has made a "target loses half its current hit points" spell. But I can't find it with a cursory inspection, so let's make one of our own.
I think it'll be a Megiddo Mehida spell, because that's most of why I introduced Megiddo Mehida into canon, so I'd have somebody whose name I could stick to the beginning of new spells, because the "[Person]'s [Adjective] [Noun]" naming convention appeals to me, it feels old school. So Megiddo's... what? Pumpkin Ray? Smashing Pumpkin? Actually, I kind of like that. Megiddo's Smashing Pumpkin.
My next thought: it definitely should allow a save, I think to negate ("you lose a quarter of your hit points instead of half your hit points" is a little weird), unless I also make it a ranged touch attack, in which case the save should halve the damage, because otherwise there are just too many points of potential complete failure (spell resistance (which it should definitely allow, because it's a magical effect applied to your target, not an object conjured up and thrown at your target); touch AC; save) for it to be worth casting.
But what kind of save? Do we want it to be a mist or a projectile or a mind-affecting spell or what? Let's try to figure out what the least-commonly-targeted save is, and target that one. I pulled up SpellForge and sorted the list of spells by save. Any given result may well be off by an order of magnitude, but I only want an estimate. The estimate is that 177 spells and powers allow reflex saves, 391 spells and powers allow fortitude saves, and 786 spells and powers allow will saves. BUT, all breaths and most traps allow reflex saves, and all poisons and diseases allow fortitude saves. So that's not conclusive. Let's preliminarily say "not a will save".
Since I think I do want to make it a ranged touch attack (to allow more points of potential failure, because "you lose half your hit points", while not half as bad as "you die" (you lose half your hit points, you're still in the fight and can easily be healed; you die, you're out of the fight and it's more expensive to heal you), is still pretty bad (though, again, I don't want to add so many points of potential failure that nobody ever uses the spell, though a homebrew spell that is too weak is more desirable than a homebrew spell that is too strong)), which already incorporates the possibility of dodging out of the way (and because Mettle is much rarer than Evasion), let's make it a Fortitude save.
Let's call it a Necromancy spell, because in my mind it fits, especially if you've played enough Ogre Battle to be familiar with Deneb, creator of the Pumpkinheads.
It's obviously a Sorcerer/Wizard spell. Megiddo Mehida was a wizard/archivist/mystic theurge, so I'm briefly tempted to add it to the Cleric list, too, but clerics shouldn't get too many blasting spells. I'm tempted to add it to a domain, but archivists don't get domains (even if they can acquire the ability to cast domain spells with a permissive DM). What other class spell lists might it fit?
The assassin's list is entirely things to aid in sneaking up on and ganking people.
Blackguards don't get many blasting spells, but they do get a number of "be a dick" spells, of which this qualifies, so that's possible. I can certainly see Megiddo being amused at fallen paladins casting his spell.
Druids get mostly support spells, but I can definitely see a druid bludgeoning something with a magic pumpkin, so that's possible.
It certainly fits the hexblade's spell list, plus they need as much love as they can get, so onto their list it goes.
The duskblade's thing is "I'm in mêlée, hitting you with a stick and also hey look spells". I'm always a little taken aback that they get ranged attacks, too. But their spell list has no real coherent theme to it at all. Sure, they can have it, why not.
I can never quite divine what the shugenja's fluff is supposed to be, so better safe than sorry and leave it off their list. Same with wu jen and spirit shaman.
Warlocks get invocations instead of spells, and there doesn't seem to be much overlap, so that's a no.
Warmages get blasting spells and only blasting spells, but no spells quite so subtle as this (compared to half a hundred variations on "you do xdy damage on a ranged touch attack" and "target dies or nothing happens", yes "the target loses half his hit points" is subtle), so I think that's a no, even though warmages kind of need a little love (a sorcerer can be a better warmage than a warmage can, even though the warmage does get Edge).
Ehh, ok, let's call it Blackguard, Druid, Duskblade, Hexblade, Sor/Wiz.
Now, what level spell shall it be? Power word kill is level 9, but it doesn't allow a save or require a touch attack (though it doesn't affect creatures with more than 101hp). Megiddo's smashing pumpkin is much weaker than that.
The 4th level spell Phantasmal Killer is a much better comparison. It instantly kills the subject. It allows two saves: a will save which cancels all effects, and a fortitude save which reduces the effect to 3d6 damage. It's also [mind-affecting], so it doesn't affect half the creatures you might expect to encounter (most undead, oozes, vermin, many plants, et cetera). Two saves (of which one negates, one reduces damage to 3d6) is comparable to a ranged touch attack and a save (of which one negates, one reduces damage to half). The [mind-affecting] tag is worth a spell level or two, if it could affect every foe it would be a level or two higher. But death is, as established above, more than twice as much of an inconvenience as losing half your hit points, so that's a couple spell levels weaker. So let's call Megiddo's smashing pumpkin oh, say, level 4.
One situation in which Megiddo's smashing pumpkin is actually clearly better than Phantasmal Killer is against creatures which are immune to death effects. But these are relatively rare, and mostly very high level, and if you're high enough level to face something immune to death effects then you've got better options than Save-or-Dies anyway.
Yes, the poor Tarrasque is a joke (lollercoaster @ Toughness 6 times), but he's the archetypical CR20 monster. So: you cast this on him, you need to defeat his SR32 and his fortitude save of +38 (his touch AC of 5 is a joke). If you do, you do 429 damage. That seems preposterously powerful for a 3rd level spell, but Phantasmal Killer would do the same thing at the one level higher but better than twice as well.
Now consider a lizard. Defeat its touch AC of 14 and its fortitude save of +2, you do 1 damage. That's a joke, even for a 3th level spell. But then, if you're casting Megiddo's smashing pumpkin on a 2hp creature, you deserve to be pointed and laughed at.
So 3rd level is fine.
It doesn't quite perfectly fit the druid's concept, so let's make it a level higher for them. Blackguards and hexblades can't cast spells higher than level 4, and those only very late in their careers, so I'm almost inclined to bump it down to 2 for them, or at least for the hexblade. But is it a little too good for a 2nd level spell? Better safe than sorry, even with the hexblade.
So: Blackguard 3, Druid 4, Duskblade 3, Hexblade 3, Sorcerer/Wizard 3.
--- Let's throw it all together
Megiddo's Smashing Pumpkin
Level: Blackguard 3, Druid 4, Duskblade 3, Hexblade 3, Sorcerer/Wizard 3
Components: M, S, V
Casting Time: 1 standard action
Target: One creature
Saving Throw: Fortitude half
Spell Resistance: Yes
A moldy, rotten pumpkin flies from your hands and beats your opponent about the head and shoulders before crumbling into nothing.
Make a ranged touch attack. If it hits, the target takes force damage equal to half his hit points before the spell's effect, rounded down (put another way: his hit points are effectively reduced to half their previous total, rounded up). A successful fortitude saving throw halves this damage (leaving him with 3/4 of his previous hit point total).
Like any attack, this spell deals a minimum of 1 damage. If the target had 2 or fewer hit points, his hit point total is instead reduced by 1.
Material Component: A single pumpkin seed.
Now that we've got that done, we could create a pumpkinman monster, for even closer adherence to Ogre Battle. There are two possibilities: the easy possibility and the hard possibility.
The easy possibility is this:
Take the standard skeleton template.
Replace its head with a pumpkin.
It can cast Megiddo's smashing pumpkin as an at-will spell-like ability (save DC wisdom-based).
CR increases by +1.
It retains the undead type, but it is susceptible to spells and special abilities that target or affect plants.
You can use animate dead to create a pumpkinman of this sort by adding a fresh pumpkin as an extra material component.
The hard possibility is this: create a custom monster. This was my original plan, and I even found a CR calculator to aid the process, but then I realized I'm quite content with the easy version and I don't know that I could improve upon it.
EDIT: I have found a spell in the Spell Compendium: Avasculate. Ranged touch attack, subject is reduced to half its current hit points (rounded down) and stunned for 1 round. Fortitude negates the stun. This is a level 7 spell. That, um, is kind of crap compared to the 4th level Phantasmal Killer. I maintain my original judgement of the proper level of this spell.