I'm looking for ways to ensure an enemy dies on the exact blow I want them to, and one of the solutions I came up with was to drop them with nonlethal damage, then convert it to lethal.
Is there any kind of effect that does this?
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While there are numerous ways in D&D 3.5 to finish an opponent in a single hit, I know of no specific ability in any official book, magazine or web article which allows a player character to "convert" nonlethal damage an opponent already took into lethal damage.
I searched for "nonlethal" at the WotC third edition archive, an unauthorized web index of official 3e content which I won't link here, the official Consolidated Lists Archive, and the D20 SRD. I've also checked the Book of Vile Darkness, which is the only likely source of a spell for converting nonlethal to lethal (the merciful opposite appearing in its counterpart Book of Exalted Deeds), but I can find none there, nor in a perusal of the DragonDex (as some Vile content appeared in that magazine too).
Non-lethal damage is specifically stated on PHB p.146 to be not real damage:
Do not deduct the nonlethal damage number from your current hit points. It is not "real" damage.
Specifically, nonlethal damage doesn't subtract from your remaining hit points, but rather accumulates indefinitely, causing you to become unconscious when your nonlethal damage exceeds your current hit points. Nonlethal damage is specifically intended to subdue an opponent without actually wounding them, and there's no way to suddenly make that into real wounds, nor any core mechanic designed around this particular rare use.
There are other ways to guarantee last hit, or at least increase its likelihood. You might coup-de-grace a helpless opponent, or attack with a poisoned weapon which deals Constitution damage, or an instant kill spell or special attack, or a readied action, or deal 50 damage in one hit to cause Massive Damage, or deal a huge amount of damage in one hit. However, based on my experience of D&D third edition and a reasonable search of the appropriate sources, there's no way to do this by converting damage's type after it was already taken.
This is different than the exact request you made (convert nonlethal damage to lethal damage) but you can (hopefully) kill someone with one blow once you dealth enough nonlethal damage.
When someone has received more nonlethal damage than their hp total, they are unconscious:
Knocked out and helpless. Unconsciousness can result from having current hit points between -1 and -9, or from nonlethal damage in excess of current hit points.
When someone is unconscious, they are also helpless:
A helpless character is paralyzed, held, bound, sleeping, unconscious, or otherwise completely at an opponent’s mercy. A helpless target is treated as having a Dexterity of 0 (-5 modifier). Melee attacks against a helpless target get a +4 bonus (equivalent to attacking a prone target). Ranged attacks gets no special bonus against helpless targets. Rogues can sneak attack helpless targets.
As a full-round action, an enemy can use a melee weapon to deliver a coup de grace to a helpless foe. An enemy can also use a bow or crossbow, provided he is adjacent to the target. The attacker automatically hits and scores a critical hit. (A rogue also gets her sneak attack damage bonus against a helpless foe when delivering a coup de grace.) If the defender survives, he must make a Fortitude save (DC 10 + damage dealt) or die.
Delivering a coup de grace provokes attacks of opportunity.
Creatures that are immune to critical hits do not take critical damage, nor do they need to make Fortitude saves to avoid being killed by a coup de grace.
And (as stated in the condition description) helpless enemies can be killed with a Coup de Grace:
As a full-round action, you can use a melee weapon to deliver a coup de grace to a helpless opponent. You can also use a bow or crossbow, provided you are adjacent to the target.
You automatically hit and score a critical hit. If the defender survives the damage, he must make a Fortitude save (DC 10 + damage dealt) or die. A rogue also gets her extra sneak attack damage against a helpless opponent when delivering a coup de grace.
Delivering a coup de grace provokes attacks of opportunity from threatening opponents.
You can’t deliver a coup de grace against a creature that is immune to critical hits. You can deliver a coup de grace against a creature with total concealment, but doing this requires two consecutive full-round actions (one to "find" the creature once you’ve determined what square it’s in, and one to deliver the coup de grace).
Since it is always a crit, a coup de grace dealt with a weapon with a high crit multiplier (like a Scythe or a Greataxe) will make an instant death very likely. In general, the more damage you deal with a coup de grace the better.
Note that (as stated in the coup de grace description) you still deal damage with a coup de grace so the enemy might just die from the attack.
However, given existing D&D rules, and with clever application of them, this can be accomplished.
The basic idea is to use Damp Power on a lot of people (which reduces incoming damage to 1hp per die) to figure out how many times it takes to knock them unconscious, and/or kill them via statistical analysis.
This process will reveal the concept of hit points, and if used against stats, stat points, to the characters and to the world at large.
After enough research has been conducted to realize that different professions produce different increases, how much healing various methods yield, and a lot of greyhawk and error... a power or spell can be created based on all of the data gathered that shows everyone's hit points, stat points, and possibly a few other characteristics, at least to themselves.
Considering that Sherlock Holmes was described as doing this sort of analysis on corpses and crime scenes, it would probably take that level of intellect to come up with the idea in the first place.
This should then enable your desired end result - an in game method of a "Status Page" power or spell that can determine the exact point of unconsciousness... after years of effort and research in-game.
This method will not, however convert non-lethal damage into lethal damage, only allow characters to become aware of hit points as a concept within the game and control how much damage one deals.