Whats the most unkillable, immortal character possible pre-epic in D&D 3.5?

Question

I'm trying to make a character that's as unkillable as possible without the use of epic feats or Wish cheese. Preferably I'd like to stay away from things like Pun Pun that just do everything and gain a divine rank at level 1.

So far my work has made something mostly invulnerable early on. The build I've made so far is a typical diet Death Knight build that uses Troll Blooded (DR 319) and Bone Knight (Five Nations) to get immunity from most damage at level 9 and immunity from most other things that can kill me at level 13. This could theoretically be done a level earlier by using Paladin 4/Bone Knight X, but I find that the added utility and power from Cleric 4/Full BaB Class 1/ Bone Knight X is worth it.

Assuming it works with the regional requirements for troll blooded in your game and that your DM allows flaws you can also make your race a Fire Gnome (Planar Handbook) for fire Immunity at the cost of +1 LA. The feat Weeded to Time (DR 354) also allows you to become immune to aging, assuming it gives the Endless trait like it implies. and Planar Chasuble (Incarnum) can gives energy resistance 10/acid. Which isn't enough to stop all damage but it adds to your durability against the only form of damage that can harm you meaningfully. Adding the Feral Template (SS) for added fast healing might also be worth it.

And this is where I've hit a roadblock. I'm having trouble finding a place to take this build from here. I need a way to gain acid immunity and a way to dodge any other forms of potential death. I'd also like to find a way to get fire and acid immunity onto a human to make this build a bit less shaky. I'd also like to keep it as functional as possible. As is, this is a feat tax-heavy build that can still make for a good Necromancer divine casting "Gish", though a lengthy dip into something that fixes most of my issues isn't out of the question.

Answer 1

A likely contender is sofawall’s “The Cube” build—except it was never posted fully in public, and at least some of its secrets have been lost. This post details some of it, saying

The Cube? That was me.

• Prismatic Wall
• Wall of Force
• Magically Hardened Obdurium
• Lead
• Dirt
• Me

It can move. As it still a build in progress, I will say little, other than the bare-bones version is available at level 9 and effectively free at level 12ish ([away from books]).

The current build is immune to almost any targeted spell (even without the walls), and any non-targeted spell near it is eaten by it. It is also much more expensive, although still within a level 13 budget, for ToS [Test of Spite] (the arena that it was created for) The biggest weakness, MDJ [Mordenkainen’s disjunction], has been defeated.

No, I will not tell how.

Amusing anecdote time! I originally made it because I was asked to make a build for the arena, but I didn't want something all complicated and junk. So I just decided to ignore class, race and skills, and only one feat was taken. Everything else was chosen by dice. It eventually was changed to commoner, just to say I won with a commoner. Signmaker is the only competitor to (sort-of) defeat it in the ToS, mainly because she just ran away until I died of old age (we ruled it a tie). I am now immune to that. Also, I have not faced PhoenixRiver's Fluffy build, the other Tier 0.5 (by the ToS tier system).

EDIT: I forgot! Doc Roc also helped me with many things, one of those being immunity to MDJ. Also, one-way LoE/LoS [Line of Effect/Line of Sight].

EDIT2: Most efficient way to kill things is simply ramming them with Prismatic Walls. As the SBG [Stronghold Builder’s Guide] has nothing to say about what DC the Wall is, I always assumed minimum. Side note, the SBG is a really poorly edited book. Making a tornado is cheaper than making a windstorm. Actually, assuming my memory is correct, you cannot make a windstorm, as the item to do that says tornado instead.

—sofawall, March 3, 2010

The Cube wound up being a mobile fortress from which the pilot could cast spells, but into which spells and attacks could not be made, including divinations and teleportation. The asymmetry made it impossible to defeat.

Notably, the protection against disjunction probably wasn’t quite absolute immunity—it seemed to involve massive amounts of dispelling screen, possibly through an overclocked spell clock. A similarly-absurd number of disjunctions could presumably overwhelm it.

Finally, note that the Cube was a challenge in Test of Spite, an arena game that was trying quite hard to push the boundaries of what was possible in 3.5e, and then to define hard boundaries beyond which things became entirely “broken” to the point they could not be stopped. Its massive banlist was built up primarily as a result of actual matches in which it was judged that a build was too successful, and removing key elements from it. That means ToS is likely the most extensively and explicitly defined setting in which we can push against known constraints. But the people running it were imperfect, and not every ruling was accurate—one of the most notorious matches, which saw the banning of the Lightning Maces feat, was ruled incorrectly, and the absurd critical hit that was determined to be unreasonable for the arena should never have happened as a result of death urge.

The other half of that, though, is that the Cube had to work within the banlist—which means it could be improved with material outside it that may yet be still “away from Pun Pun.” For example, dweomerkeeper, a prestige class from the Complete Divine web enhancement, is banned in ToS because its ability to turn any spell of your choice into a supernatural power is patently absurd. But if you had that ability, then you could make supernatural prismatic walls—which might be immune to disjunction. See the end of this answer for a discussion of disjunction against supernatural effects (the short answer is, supernatural effects “are disjoined,” but what that actually means for them is unclear and undefined). There may be other options.