I am curious if this powerful epic spell has ever been included, in some form, in a NWN or NWN2 module. For those who aren't familiar with this spell, it (if the caster succeeds with a DC 419 Spellcraft check) deal 305d6 damage (Fortitude save for half) to the target and 200d6 points of backlash damage to the caster.

I am curious about whether anyone ever implemented this spell. (Of course, there's the small problem of getting your Spellcraft bonus high enough to cast this spell, and the problem of surviving the backlash (or maybe it's used by an NPC who doesn't care about surviving the spell).) http://www.d20srd.org/srd/epic/spells/vengefulGazeOfGod.htm

I can't say for sure, but I think it's unlikely. To acquire this spell, you need to develop it and pay all of the development costs, including XP costs. As I understand the rules, you cannot spend so much XP that doing so would cause you to lose a level. If that's true, then since the XP cost for developing this spell is 150,840 XP, that means you would have to be at such a high level that it would take at least that much XP to get to the next level, before you could acquire this spell. The Player's Handbook doesn't list levels that high, stopping at level 20. The Epic Level Handbook lists XP levels up to 30, and even at level 30 you can't spend that much XP, because it only takes 29,000 XP to go from level 29 to level 30.

Of course, though this may not be a spell for players at all, but for NPC gods to use to smite mortals, I don't know of any modules that have implemented characters like that.

There's nothing inherent to it that would make it difficult to script, though. It's just having a character capable of using it that would be the issue.

The epic spell rules work differently in NWN than in Pen and Paper than they do in NWN. In PnP you pay one feat for access to all epic spells, but you learn them by paying gold and XP (although there's a well-known exploit to reduce the development cost and time of a custom spell to 0). In NWN it's one feat per spell with no development cost. In addition, those spells that were imported over were often buffed substantially. For instance, Hellball requires a DC 90 spellcraft check and 200 XP to cast and has a 10d6 backlash, which needless to say is laughably bad.

However, most people people consider the epic spellcasting rules from pen and paper to be so broken as to be useless. A lot of the pregenerated examples like Hellball or Vengeful Gaze of God are just unusably bad, but the system is so freeform that there are countless loopholes or opportunities to create feedback loops that you can completely break the game. Want to create an army of perfectly loyal clones of yourself? It's relatively easy, actually. From what I've read, almost no one actually uses the epic spellcasting rules in PnP because of how irredeemably dysfunctional they are.

Epic-level play is generally frowned upon in PnP circles, and the only reason it has any following whatsoever can probably be attributed to its success in NWN and NWN2. And a big part of that success was that they did their own thing rather than adhering to the terrible pen and paper rules. The Epic Level Handbook was one of the biggest flops of any 3rd edition product. Wizards of the Coast basically abandoned epic-level play after that fiasco; it would never receive any official support or recognition going forward. They didn't even bother to issue guidelines for adapting the epic level handbook for 3.5.

They did, for free:
D&D v.3.5 Accessory Update booklets
http://archive.wizards.com/default.asp?x=dnd/dnd/20030718a

Huh, could have sworn I read someone remarking that they never even acknowledged its existence in 3.5

Well, my apologies for spreading incorrect information.

I have actually studied the Epic Spellcasting rules, and even without actually running it, I can tell that the seeds aren't balanced. Fortify is overpowered (except when used to boost HP), while Destroy is underpowered. (Basically, I see a +1 DC increase to be about the equivalent of 1 non-epic caster level.)

If the Destroy seed were balanced more sanely:
The DC would be 285 points less, putting it at 134. Suddenly, the spell looks a lot more reasonable now.
Get rid of the backlash, and the DC is now 334, which is at least close to what a level 305 spellcaster should be able to manage. (Given the general rule of 1d6 per caster level for attack spells, level 305 seems about right for this spell.)

Even still, I think the spell would be underpowered for a level 305 spellcaster, as it can be resisted, is save for half damage, and is only single target. (Consider that a level 305 character with semi-decent (for this level) Constitution will be able to easily survive this attack.)

Note that the epic spell system also depends too heavily on the skill system, which is too broken to use for anything important; basing it off caster level would be better.

Anyway, I think the problem with epic levels is that D&D 3.x doesn't scale well to high levels. I can think of two examples of this:
1. If Constitution grows linearly with level, then HP grows quadratically; a character of double the level will have quadruple the hit points. Spell damage only grows linearly.
2. The accuracy mechanic doesn't scale well. A mere 40 levels can mean the difference between almost always hitting and almost never hitting. In other words, around level 4000, a mere 1% difference in levels can mean the difference between an easy victory and having no chance whatsoever. (Contrast this to Disgaea, where a 1% difference in levels is no big deal.)

I actually do like the idea behind the epic spellcasting system; it just needs to be better balanced. (I note that the Elder Scrolls series has a similar system (except in Skyrim, which removed it); it's fun to play around with, but unfortunately, there are some balance issues with it.)

You hit a nail on the head there. Skills scale differently from other kinds of numbers, and tend to be at the heart of a lot of horribly broken things. Using them as a restriction here was a bad idea. However, you're dead wrong if you think you need to be level 300+ to manage a DC 334 check. Consider the Jumplomancer, a 15th level build that manages a whopping +370 skill check on jumps and uses a convoluted combination of abilities to use his jumping skill to turn onlookers into fanatically loyal followers. And while spellcraft is a bit harder to increase than jumping, I'd be very surprised if you needed to be higher than the mid-30's to make that check.

But in the same vein, 305d6 actually isn't very high by epic standards. That only averages somewhere around 1000 damage, and those kinds of numbers are doable on non-epic builds. Remember what I just linked to for jumping? You can do the same sort of [url=http://www.giantitp.com/forums/showthread.php?447435-quot-The-Mailman-A-Direct-Damage-Sorcerer-quot-(from-Wizards-forums)]stuff for damage[/url]. This is the reason why spells that deal direct damage are disliked in PnP. They tend to fall behind the curve long before the epic levels. Spellcasters that do use damage-dealing spells will specialize in metamagic, which doesn't work with epic spells, since it's the only way to get big multipliers. That's more of a NWN-centric perspective. In PnP there are a lot of ways to bypass a high AC, and battles are won and lost by spellcasting superiority more than anything. After all, it doesn't matter much if you have higher AC, attack, and damage if you can't make attacks in the first place. I think a lot of people would agree with the first part, but based on my reading the second part verges on impossible. I've lurked on pen and paper boards for the past several years, and the creatively brilliant ways people utterly break the game is awe inspiring.

This also makes me wonder why WotC changed Delayed Blast Fireball and Horrid Wilting from 1d8 to 1d6.

(There's also the fact that single target spells don't appear to do any more damage than multi target spells, even though multi target spells have the clear advantage of hitting multiple targets.) If you can't be hit except on a natural 20, you have a chance of surviving long enough for the spells to wear off,

Also, this same issue applies to saving throws, and even more to spell resistance (where you need only a 20 level difference to have this effect). Then again, I personally dislike the Spell Resistance mechanic; aren't saving throws enough? (The SR mechanic also favors spells that don't target opponents over those that do.)

A better accuracy mechanic would be to take the attacker's accuracy and divide by the target's evasion to determine whether an attack hits; this way, a 1% difference will not have much of an effect even if the stats are in the millions. (Of course, this algorithm would pretty much require a computer or calculator, as it would be too cumbersome to do the division by hand every time someone attacks.)

Doesn't help if the spell's duration is measured in hours, or if the enemy has a way of attacking you that bypasses AC and saves altogether, or if the spellcaster just does something to massively lower your defenses, or attacks you indirectly (ie, remove all air from the surrounding environment and let you suffocate). This is why spellcasters love spells that don't allow for saves or spell resistance. Their options certainly are limited if the opponent's saves and SR are imperviously high, but if there's one thing I've learned in my years of lurking is that no matter how convoluted or overwhelming a defense you present there's some spell or spell combination out there that will beat it. Division would definitely be toxic. Epic-level play is bogged down enough as it is just with the sheer number of dice rolls and big numbers to track, it'd slow down to an unbearable crawl if the math was much more complex than simple addition and subtraction.

However, that same additive nature of the game also works with levels. When determining relative power you're not supposed to divide, you're supposed to subtract. The power difference between level 5 and 6 is supposed to be the same relative power difference between level 15 and 16, or level 25 and 26, or (hypothetically) level 4005 and 4006. Now, in practice this was only ever true in the broadest of strokes and breaks down completely in the epic levels, but at least in terms of the design that was the idea.