This document contains massive spoilers for the story
and gameplay of Heroes. It is intended to lay out in detail
the story that was designed by the author; whether that
story is actually visible and figure-outable from the
game itself--that is, whether this was successfully
incorporated--I leave you to decide.
References from the game are notated in square brackets,
with a one-letter abbreviation for which character it
appears in (a for adventurer, e for
enchanter, r for royalty, t for thief,
d for dragon, and * for all characters).
The group consisted of:
(Thorn might well have been a paladin, given his behavior throughout the story, but this is pure guesswork.)
Merresin had believed herself to be Thorn's lover, but whether she was or not is disputed [e: X BLACKHELM, r: epilogue]; given Thorn's reaction [final epilogue], it seems likely it was unrequited.
On an adventure, two of the companions--Thorn and El, the bard--discovered a crown which made its holder king/queen of the lands in which the story is set. The bard murdered Thorn by pushing him off a cliff [r: epilogue, the final epilogue, and also by implication of the fact that what s/he describes would presumably not be murder, but there is clear evidence of murder, described later].
On Thorn's death, Thorn's Companions disbanded [a: ASK BRANNA ABOUT THORN], and El the bard presumably took the throne.
The dragon had been up to something which we don't know, but apparently whatever it was it was not what it was supposed to be doing, and the dragon elders assigned him the task of guarding a treasure known to humans as The Dragon Gem and to the dragons as the Kthyress Crystal [d: prologue].
El, the new Royal Crown, discovered there was a powerful spell known as The Way of the Grave which could be used to bring a murdered person back from the dead if the murderer was at hand [a: epilogue] and which required an object called a Kriit [t: epilogue], which is a copper pole with a mechanical claw on the end [e: EXAMINE KRIIT]. Fearing that someone might decipher Thorn's murder, El outlawed the ritual except if s/he presided [a: epilogue] and presumably had all the Kriits destroyed (assuming there was more than one) and the remaining one secured in the Temple of Justice [r: ASK TEMPLE GUARD FOR POLE]. (By the time the game takes place, El has forgotten the details of what the copper pole is and what it is for.)
Fifteen years ago, Merresin discovered that Thorn had been murdered, rather than experiencing an accidental death. [Hypothesis from the final epilogue]. This apparently led to conflict between Merresin and the other party members [e: CAST any spell ON GEM while the trap is active; the fear/distrust of Merresin is in all the prologues].
We speculate that Merresin wanted help bringing El to justice, but that the other Companions refused to help her. At some point, perhaps then, perhaps earlier, El determined that she was a danger, and El set out to have her elimitated [r: initial room description].
At some point, Merresin determined that she could use The Way of the Grave to ressurect Thorn. Through Sedmon, she tried to hire Yel, the thief, to acquire the Kriit, but s/he refused [t: epilogue]. Apparently Merresin was unable to recover it herself, although it is unclear why; perhaps she wanted to avoid El realizing that she was the one who had it. Or perhaps she wanted a form of vengeance by getting one of her former party members involved; perhaps she just liked manipulating people.
In the end, she set a much more subtle plan in motion. The Kthyress Crystal possesses a "geas of greed" [d: X CRYSTAL; *: X GEM (each of the characters reveal a greedy attitude towards it, and an intuition that everyone would want it); also, * epilogue, where each character shows a slight obsessiveness towards it].
Merresin fed rumors to the thief about the Dragon Gem, relying on the Gem's powers to encourage the thief to take a chance s/he presumably would not otherwise have done. [t: prologue suggests Meressin's involvement.] Presumably she secretly cast some sort of spell on the thief to allow him to penetrate the lair of the dragon [d: prologue 'certain magics'].
She then had Sedmon acquire the Dragon Gem from the thief (t: prologue), and Sedmon spread the false rumor about having slain the dragon. (One suspects that Merresin was manipulating Sedmon without him being aware of it, and hence the Dragonslayer rumor might have been purely his own doing.)
She summoned and bound an imp [epilogue] and cast him in the role of Sedmon's Guard [d: SMELL GUARD and epilogue; other evidence about this guard is the resistance to the enchanter's spells, and things like r: ARREST SEDMON]. She also arranged the world into the situation which is at hand at the beginning of all of the stories. (The only significant differences in the state of the world in each of the stories is that in the royalty's story, people have advanced word of his/her approach, so the Baron is with the gem and the temple guard comes out to meet the royalty.)
She then leaked word that she was going to acquire the Dragon Gem from Sedmon, and sat back to see which of the five characters might bite [prologue, the telling of all five stories]. Whichever it was, she had taken pains to make her interest seem to only be in the Gem, not the Kriit (except for the dragon, whom she rightly judged would not care what she did). In some cases, that meant she went to great effort to continue the charade even after she or her minions had the Kriit, e.g. with the enchanter.
After she acquired the Kriit from one of the "heroes", she then performed The Way of the Grave on El and Thorn--only to have Thorn reject her yet again.