Alright then ...
After several years, the Great Grungy Swamp had become full of life again, life adapted to it's extremes of topography, climate, and adverse conditions. Quasi-magical trees were sprouting, and the swamp floor and marshes were covered in greenery.
The elves rebuilt homes around the ruins of Haldendrea in their new image, creating Mordenkainen's Mansions with all the lack of comfort of home (read: it was as much a cesspool inside as out.) The elves had grown to love their new environment, to think of it as home.
Even though they had regained access to arcane magic, and their divine magic was reasonably strong, and they could have altered the local area back into a truly habitable and nice place again, they did no such thing. They were quite content with how things were.
The number of monsters in the Swamp (the elves had heard of the new designation for Delrune - the Great Grungy Swamp - and were much amused) had skyrocketed.
The elves were in constant battle with these monsters. Life was constantly painful and harsh. Life was a crucible that the elves lived and breathed.
Thanks to the RAW, thanks to the D&D reality where overcoming adversary gains one experience and levels, the elves grew stronger by gaining levels. In roleplaying terms, they became more and more battlewise, more and more skilled at war, ever more accustomed to war and all it's horrors.
Their will to live ever increased, until they begin to actually glow with life force to those with eyes to see. Their mental fortitude and spirtual fortitude ever increased. They became harder to kill (the game mechanics altering more and more from the norm, the elves able at this point to go to -10 and remain conscious, and to -20 before they died.)
They metamorphosed into staunch, ferocious warriors, each and every one of them, regardless of any class taken, regardless of gender, regardless of age (except for the truly young.)
Vecna was still off fighting in Greyspace. His Legions were out with him. Subservient nations were arising in the shattered Flanaess under his rule.
One of these was the illithid nation of Isyrium, south of Haldendrea where Veluna and western Furyondy had been.
Illithid scouts penetrated the Swamp and ran into several elves. The elves killed them. Perceiving this new threat for what it was, many of the elves embarked on efforts to develop their psionic potential. A few of them proved to have that potential, and began to explore it.
At this point, the elves looked at the situation, and saw how grim their predicament was. The illithid would surely send more scouts, and eventually they would be discovered. That news would be carried back to Vecna, and then they would suffer the fate of all their brethren.
The elves had evolved enough arcane magic, or rediscovered enough in the ruins of old Delrune, to simply leave by creating a Gate.
The elves, elected to stay.
All 3,000 of them.
They threw a Ritual. On Toril, it might have been called a Mythal, but Mythals tend to affect areas, and this affected only the elves themselves. It enhanced certain behavioral traits, inhibited others, and prohibited certain behaviors altogether.
It prohibited:
- Elves killing elves (while allowing them to magically recognize other elves), harming other elves, or plotting any action deliberately intended to bring any harm upon an elf.
- Elves permanently leaving the community (it limited how long they could stay away at all.)
- Elves knowingly betraying the community (if unknowing, it killed them before they could betray.)
- Elves refusing to aid the community.
It inhibited:
- All prejudices towards other elves in the community for their ways
- Any tendency to look down on other elves in the community for any reason
- Any tendency towards verbal sparring or verbal aggression between elves of the community
- The ritual prevented elves, through the combined might of all of them, from being sucked in and devoured by evil magics and dark forces. (In effect, it served as a rope from which a cliff climbing elf could descend, right down into the depths of What Elves Were Not Meant To Know.)
- The ritual granted the elves telepathy among the community, at extremely high speed (a hundred times faster than human speech, and the ability to send images, points of view, and deep thoughts and feelings.)
- The ritual fortified the elven will to live further, bostering that will with the backing and strength of the entire community.
- The ritual conveyed constantly, endlessly, the value of other elven lives in the community, creating a situation where the well-being of one's fellows was of epic value.
- The ritual attempted to strengthen certain personality traits.
In honor of this ritual, which was successfully cast, the elves renamed their city Haldendreeva (the City that Transcends.)
But the ritual did not work out the way the elves intended.
For over a decade after the encounter with Isyrium and the subsequent ritual, no civilized force entered the Swamp, and the elves remained undetected by Vecna's Legions. Vecna finished his war of conquest, was annointed at Greyhawk City, then went off to work on further goals (to unseat the Lady of Pain in Sigil.)
But hoards of monsters made the Swamp their own, large numbers of undead emerged from the blood of the fallen, and the Elves of Haldendreeva were up to their ears in war.
The war changed them, bit by bit, into something unrecognizable.
They immediately started Raising their fallen after the Ritual. Because of the lifefire among them and the Ritual, the deceased desired to return. Better Resurrection spells were come by over time, making Resurrection easier until the elves finally perfected a Resurrection spell that cost no experience to cast.
After that, Resurrection was automatic if they could find your body. Then, you started leaving a lock of hair in case your body wasn't found. Then, your lifefire grew so great your Constitution could not drop below 10 from Resurrections. Finally, your Constitution could not drop at all.
After that, life and death lost their meaning. One merged into another and into another.
War stopped being war, and became a game. Because of the Ritual, you played it to the utmost of your ability and strength, and the Ritual drove you harder and harder with the passing of time, not less and less.
You stopped being afraid of dying or capture or torture. They became normal. You inflicted them as normal on the foe. Life lost all meaning except the meaning given by the Ritual, to fight for the elves, to fight for the community, to fight on endlessly, to go on fighting forever. To literally go on fighting forever.
You still cared about your loved ones. You still cared about your community. But you lived in an abstract world where war became life, where they were one, and if there was not war the idea of still being alive was unimaginable.
With this attitude becoming ever more prevalent and ever more pronounced, then extreme, the Elves of Haldendreeva took on all comers and won. As they won, they (within the RAW) gained more levels and hence more power and understanding. With childish glee, they turned greater power and understanding into greater war.
Then even this was not enough. The elves discovered a magic that would transform other beings into any other being, with all it's knowledge and skills and spells (if applicable) retained. They started using this to turn powerful enemies into Elves of Haldendreeva, who in turn were powerful. The Ritual immediately ensnared these new elves, made them over, and infused them with the particular madness of their new kindred.
Sometime late in this phase of their development, the elves discovered Lifeproof, and Lifeproofed their entire community in a matter of weeks. It is thought they used chronomancy, and actually it took them years to accomplish this goal. But the elves had made their first break throughs on magical and psionic longevity, and age itself was no longer a factor.
The RAW made the madness pay off, since D&D is a game based on killing.
Even the 3rd edition RAW for experience paid off, since as a general rule the elves hid from the bigger encounters until they were ready for them.
With each new high level spell or high level psionic power gained, the elves turned reality on it's head once more. With each new gain, they gained the power to practice their madness more. And practicing their madness just kept on making them more and more powerful, with more and more insight and understanding into becoming ever yet more powerful.
And still Vecna could not be bothered by the strange reports from his spies.
Something was happening in the Great Grungy Swamp. Something involving a lot of magic. Something scouts were not finding, because they never came back. Something involving elves.
Vecna dismissed the reports. Then grew irritated at further reports. Then gave his minions leave to go in and scour the Swamp for pests (he had decided no elves actually existed ... no elves could have survived in there, but some joker was trying to convince him of the impossible.)
Finally, a very annoyed Vecna, intent on his plans for overthrowing the Lady of Pain in Sigil, authorized his servitors and servitor nations to do something about the pests.
But Vecna himself still did nothing, spending every last minute in other plans and preparations, barely sparing a thought for the Swamp and it's problems.
It was now 18 years since Vecna had leveled Delrune.
At this point, every elf in Haldendreeva (many thousands strong now) was 17th level or higher. Many were in the lower epic levels.
All were quite insane. An insanity they had deliberately chosen, had deliberately inflicted on themselves, because it was the only way (other than flight) that they could survive.
The RAW for D&D and a conception born of frustration, made it all possible.