Wirt's backstory in the original Diablo is by far the saddest of the bunch. He Used to Be a Sweet Kid until he was taken from the arms of his mother by the Fallen, the very first demons you encounter in the game, who spirited him and other kids into the labyrinth below Tristram Cathedral. There, Wirt and the other kids were subjected to horrendous torture by the sadistic Fallen. Wirt was the only survivor of the horror, and he escaped from the labyrinth with the help of Griswold, one of the survivors of the party who followed Archbishop Lazarus into the cathedral and who is now the town's blacksmith. Wirt passed out upon arriving in Tristram, and his leg was so damaged by the Fallen's torture that Pepin, the town healer, had to amputate it and replace it with a wooden leg to save his life. When Wirt came to, he was shocked and angry, and it would only get worse when he found out that his mother had died of grief in his absence and that the rest of the townsfolk had done nothing to help him and the other kids. Is it any wonder the kid grew up as he did?
Farnham, in that same vein, is the designated town drunk because he's the only other survivor of the group that met the Butcher. Anything that reminds him of what he saw reduces him to a terrified mess, and he drinks to numb the psychological pain of seeing the corpses of his friends mutilated and strung up across the walls.
Tristram has gone through hell and a half, with the implication in the lore that you're seeing the outskirts of a larger town — and, by that count, you only see what's left of the town as a group of survivors that have little left after King Leoric's maddened reign and death, and is now standing on the precipice of a demonic incursion in their own backyard. And despite the heroism of our trio of classes fighting straight into the depths of the Cathedral, it wasn't enough; thanks to what happened to the Warrior, the Hell Gate was not truly closed shut, so shortly after everything seemed well and good, everyone was slaughtered by demons pouring out of the Cathedral anyway, short of Deckard Cain who was tortured for kicks.
What happens to all of the three playable classes in the lore after the events of Diablo could definitely count as this. From top to bottom:
The Warrior, the class who deals the killing blow on Diablo in this game. Unfortunately for him, he also winds up inadvertently killing his younger brother Albrecht, due to the poor boy being chosen as the Lord of Terror's mortal host. Seeing such an event no doubt had to be traumatic for the Warrior, but the nightmare had only just begun. So great was Diablo's influence from his Soulstone that the Lord of Terror was able to effectively brainwash the Warrior into plunging the artifact into his own head, under the guise of containing the demon's evil. Now wrestling with the demon in his mind, the Warrior would eventually go eastward, adopting the new moniker of the Dark Wanderer. In time, Diablo would prove more than a match for the Warrior, eventually making him into a host for the demon, just like he had done with Albrecht. It would not be until the end of Act IV of Diablo II that Diablo, and the Warrior with him, would meet his end, at the hands of a group of heroes who joined together to hunt him down. Thus, the Warrior is the last of the three heroes to fall, joining the companions who had accompanied him in Tristram in death.
The Rogue. It's revealed early in Act I of Diablo II that while the Rogue did indeed return to the Sisters of the Sightless Eye after Diablo's defeat, she brought along an influence of evil origin with her. That same influence was enough for Andariel to corrupt the Rogue into a servant of evil. The second quest of Act I specifically involves putting down the Rogue character, given the moniker of Blood Raven, thus making her the first of the three heroes to be given death by the same group of heroes who had joined together with the purpose of hunting down the Dark Wanderer.
The Sorcerer. Scarred by the events that had happened under the grounds of Tristram, the Sorcerer would travel to Lut Gholein, in search of the mystical Arcane Sanctuary of the great mage Horazon. It's there that he would find an old comrade of his by the name of Drognan. The Sorcerer spoke of the events he'd experienced while battling against Diablo's forces to his comrade, but such were the man's ramblings that Drognan had dismissed much of what had been said. Later, the Sorcerer would manage to talk his way into the palace of Lut Gholein, and it is indeed within the depths of the palace that he would find a gateway to the Arcane Sanctuary. When the same group of heroes that had taken care of Blood Raven reached the Arcane Sanctuary late into Act II, they would find that the man who was once the Sorcerer had now become the Summoner, a pitiable madman who believed that he could summon evil forces to bend to his will. Alas, it is evil who had bent the Sorcerer to its own will, forcing the group of heroes to slay him as the fifth quest of Act II. And thus, the Sorcerer is the second of the three heroes to fall.
