As a follow up to this question: is it possible to deal non-lethal damage even after dealing an enormous amount of damage to a target in a single turn?
[RPG] Can phenomenal amounts of damage still be non-lethal
It's actually quite simple to do this. In the SRD, you'll see a rule that allows you to deal nonlethal damage with any weapon that normally deals lethal damage:
Nonlethal Damage with a Weapon that Deals Lethal Damage
You can use a melee weapon that deals lethal damage to deal nonlethal damage instead, but you take a –4 penalty on your attack
... which makes sense. You have to take much more care not to kill someone with a sword and just knock them out. But the option is there for you! And it makes for very cool character fluff, in my opinion.
Also, as @GMJoe kindly pointed out, there is a core enchantment in the 3.5 Dungeon Master's Guide (pg. 225) called Merciful for only a +1 bonus. You'll certainly want this (assuming you don't use one of the weapons which already deals nonlethal damage):
The weapon deals an extra 1d6 points of damage, and all damage it deals is nonlethal damage. On command, the weapon suppresses this ability until commanded to resume it. Bows, crossbows, and slings so crafted bestow the merciful effect upon their ammunition.
Other Source Books
The best sources for nonlethal damage in books outside of core come from the Book of Exalted Deeds:
- pg. 34, Nonlethal Weapons: Contains, as you might expect, a broader range of weapons which only inflict nonlethal damage. Can be upgraded per the normal magic item rules and you don't need to take the -4 attack bonus associated with the above rule. Includes...
- Truncheon: 1d8 nonlethal, bludgeoning, martial, one-handed melee weapon
- Entangling Pole: 1d4 nonlethal, piercing, exotic, two-handed melee weapon
- Grasping Pole: 1d4 nonlethal, bludgeoning, exotic, two-handed melee weapon
- Lasso (yee-haw!): special, --, ranged weapon (10 ft.)
- pg. 40, Feats: There are a couple of feats which will allow you to deal nonlethal damage more effectively. Again, all of these come at the cost of the feat, so they will still essentially make you less effective than someone doing lethal damage (who at least has one feat up on you).
- Subduing Strike: No penalty when dealing nonlethal damage; rogue’s sneak attack deals nonlethal damage
- Holy Subdual: Transform bonus and smite damage to nonlethal
- Nonlethal Substitution: Transform spell’s energy damage to nonlethal
- Vow of Nonviolence: +4 to spell DC if it does not deal real damage
- Vow of Peace: calm emotions aura, +2 natural AC, +2 deflection AC, +2 exalted AC, some normal weapons shatter when they contact you, +4 Diplomacy
Optimal Build (?)
So, really, it looks like your best bet would be to become a traditional, overpowered spellcaster, take the Nonlethal Substitution feat (which will allow you to use your spells to deal nonlethal damage), then use the various Vow feats to increase your other statistics. That looks like maximum leverage to me. I don't know of any other source books that would really contribute much to this effort.
As far as dealing more damage than a normal damage dealer, that is likely next to impossible without being very, very creative. For starters, there just aren't that many sources of nonlethal damage in the core rules. If you look in the Environment section, it might be possible for you to make some homebrew rules with your DM based on these (i.e., cold effects which numb instead of kill). If you want to stay strictly within the RAW, though, I think your best bet for having a highly functioning nonlethal character would be to have a spellcaster of some kind who only utilizes sleep spells, hold person spells, and the like. I didn't include this in the main answer because these effects don't technically deal damage, but it's an option which may appeal to you.
Yes, you can tell
When running a combat, make sure that you describe nonlethal and lethal damage differently. The distinction should be clear—both in the players’ imaginations and on their character sheets.
Does this only apply to damage the characters receive?
Personally I am happy to take what the rulebooks say at face value: "make sure that you describe nonlethal and lethal damage differently" is pretty unequivocal. Some people, however, like to keep their players in the dark about stuff like this.
With all due respect, these people are wrong because Poor Communication Kills and the role of the DM is to give the players adequate and sufficient information for them to make informed decisions about their characters that have reasonable consequences. This is called agency and it is the entire point of any RPG - if you want a game without agency Snakes and Ladders is cheap and has a much shallower learning curve. I have written about it in My PCs have a plan that will get them all killed; how and why should I save them? Playing games where you are denied the right to choose is not fun!
With respect to the specific example, if you keep this information secret then the players will have their decision making ability constrained to their detriment. The fight will be tougher (probably) than it would have been and result in more expenditure of resources by the party and possibly unnecessary casualties. This will mean fewer encounters before the party needs to rest and less fun all around.
If instead, you say, "You strike solidly with your sword and open a wide gash. You recoil in horror as the green muscle tissue writhes and begins to knit back together and the severed veins grope towards one another like some macabre game of blind man's bluff" the players have been given information that things are not as they expect. They now have more agency, more choices available which broadly speaking are to persist in what they would normally do or do something different. Now, they may make a good choice and save some resources or they may not but you will have enabled instead of disabling their agency.
There are a few rules that might apply here. The first is the Instant Death rule on p. 197 of the PHB:
Note that this is primarily a rule for player characters; the default rule for how damage affects monsters is on the next page:
Since most of the time monsters and NPCs are dead as soon as an attack takes them to 0 HP (recalling that there are no negative HP in 5e), the instant death rule doesn't apply to them.
Finally, there is the rule for Knocking a Creature Out, also on p.198:
So, normally when a monster takes damage that would reduce its HP to 0, it dies. If it's from a melee attack, you as the attacker have the option of just knocking them unconscious. The amount of damage isn't a factor at all, since it doesn't apply to monsters or NPCs.