The first time we encounter Achilios, he and Mendeln are investigating an artifact he found with some sort of runic language written on it. Upon touching the object Mendeln feels a sense of infinite knowledge, whereas Achilios only gets a sensation of death. He accompanies Uldyssian after he is forced to leave Seram and is an irreplaceable member of their group. It is made apparent early on that while Serenthia is interested in Uldyssian, Achilios is very interested in Serenthia. Throughout Birthright Achilios is consistently helping by scouting ahead, hunting for meat, and slaying Peace Warders.
In the city of Partha, Achilios holds off an attack by Damos, the morlu leader, by shooting him in the eye even while his bow had been enchanted by Malic (in the body of Master Ethon) to miss. Soon after, Mendeln came to his rescue with a necromantic spell to slay the enormous Damos. A bit later in the story we discover that Achilios' complete adoration of Serenthia has paid off, and she finally returns the feelings.
Achilios helps Uldyssian in the fight against Lucion, landing an arrow through the demons throat. His second shot is caught by Lucion, and the third is magically turned against him landing in the same spot his first arrow hit the demon - his throat. This immediately kills Achilios, but after the battle is finished Mendeln suggests they bury him, and both Uldyssian and Serenthia agree. Mendeln shows them the perfect spot which happens to be very near an artifact he earlier discovered.
Scales of the Serpent
Mendeln later (in Scales of the Serpent) feels something is amiss and journeys into the forest. He is confronted by a shadowy figure whom he first mistakes as Lilith, but later realizes is actually Achilios raised as an undead. We later discover it is Mendeln's link to the Balance that allowed him to bring Achilios back from the afterlife:
- You are the one who brought Achilios back, Mendeln. Your gift and your link to the realm of afterdeath are what enabled him to return
Achilios favors staying in the shadows as he fears what Serenthia might think of him should she see what he has become. Mendeln is the only one to know of his presence for quite some time because of Achilios' fears of Serenthia's rejection, but also because Rathma claims it could lead a catastrophe.
This is also the first time Mendeln sees Rathma, and Achilios (having apparently been in on the plan) holds him in a death grip so that Rathma may safely teleport them to Trag'Oul's plane of existence.
During the battle at Hashir, Achilios blocked the back exit so that no members of the Triune might escape. After slaying a few fleeing priests he was confronted by the Thonos, a tentacled demon summoned by Lilith, and was quickly overpowered. The Thonos grasped Achilios' limbs and was determined to tear him into pieces. Suddenly a golden light appeared warming the surroundings and searing the flesh of the demon. The Thonos fled back underground and Achilios was rendered unconscious. He "awakens" two days later and begins tracking Lilith, who is now possessing Serenthia's body and leading the Edyrem. We are also made aware at this point that Achilios' arrows have been imbued with magic by Trag'Oul himself.
When Uldyssian finally confronts Lilith and tries to regain control of his Edyrem as well as banish Lilith from Serenthia's body he is betrayed Romus, by one of his most trusted. As Lilith and a dozen hand chosen treacherous Edyrem are working on a spell against Uldyssian, Achilios buys time for the Edyrem leader by slaying a number of Lilith's loyal. Gradually, Uldyssian loosens his bonds, breaks free, and lets loose pure force throwing the tainted Edyrem into the walls. Achilios then shows himself to the group, as they were unaware of his presence, and wills Serenthia to "wake-up" so that Lilith might be forced out of her body. Surprisingly, this works and Lilith flees. As Serenthia awakens she remembers seeing Achilios, but thinks it only a dream. Uldyssian corrects her but Achilios is already back in the shadows fearful of what she may think of his appearance. While debating with himself whether he should continue hiding his presence from Serenthia, or show herself and confront what she may think he is startled by a blinding light. A light very similar to the one that sent the Thonos fleeing. The being has an overwhelming presence like that of an angel, or Trag'Oul, and tells Achilios he needs him. Achilios, after sending two arrows through the being with no effect, collapses and blacks out.
Soon after, Rathma and Trag'Oul heatedly discuss their inability to find Achilios and what it may mean for Sanctuary. Inarius may have discovered the archer, or possibly some other angel. If it is another angel, it would mean the High Heavens have discovered Sanctuary, and are most likely on their way to destroy it.
In the battle in between Istani and Kalinash, the Edyrem are taken by surprise as Peace Warders cloaked by magic close in on them from their front and back. As if that is not enough, the Thonos appears again and overpowers Uldyssian attempting to crush him. At just that moment Achilios appears, his eyes blazing white and firing arrows, each one successfully impaling an eye of the Thonos and exploding soon after. He doesn't allow the Thonos to burrow and easily defeats it, freeing Uldyssian to help his army of Edyrem and eventually go on to defeat Lilith.
The Veiled Prophet
In the woods away from the Edyrem army, Mendeln is raising spirits of dead farmers to interrogate. He and Uldyssian want to discover what provoked the mere farmers into attacking an army of magically inclined Edyrem. They easily discover the farmers have been brainwashed by Inarius, but not soon after Uldyssian is shot by a dirt encrusted arrow. Mendeln fears it struck him in the heart but Uldyssian heals himself and is fine. They know it is Achilios' arrow, but they also know he is being controlled by another. Later, Achilios is commanded to kill Serenthia by his angel master and unable to choose for himself fires upon her. He misses her heart just barely and Mendeln blocks the arrow from hitting her altogether. Achilios enraged by the angels commands to kill his closest friends, boasts victory after Mendeln somewhat blocks the arrow from killing Serenthia. The angel quickly reveals that it was Achilios' strength of will that caused him to miss his target, just as he missed Uldyssian's heart:
- ...YOU MISUNDERSTAND. SHE LIVES, BUT IT WAS NOT HE WHO SAVED THE FEMALE. THAT WAS YOU ARCHER, JUST AS BEFORE...
- ...I did it? I beat you then! Kill them... kill them both... you commanded... but I didn't...
- ...NO. YOU DID NOT,... AND THAT MAY CHANGE EVERYTHING.
Later, Mendeln is surveying some ruins away from the army of Edyrem and sees Achilios. He immediately considers himself under attack and begins casting spells to hurt or kill Achilios. Achilios fires arrows and purposely misses three separate times to show Mendeln that if he wanted to kill him, he could have done so on three separate occasions. Finally, after the third arrow is fired, Mendeln realizes he is only alive because Achilios wills it so. It is at this point, the angel commanding Achilios reveals himself to Mendeln and explains that he no longer wishes to slay them. He in fact claims he wants to help them, making them privy to the fact that Diablo himself has come to Sanctuary.
Soon after, Serenthia is awoken by what she thinks Achilios' voice. She gets up and seeing Achilios through the trees, decides to follow him out into the jungle. They embrace and Achilios explains to her why he has kept hidden all this time; because he feared how she would react, and later because of what the angel was making him do. At that moment the angel shows himself to her, and explains that Diablo has come to Sanctuary and shows her a vision that he claims is the future. The vision includes Malic, in the body of Amolia, betraying the Mage Clans in the name of Uldyssian and later possessing the body of Uldyssian. Because of this vision, Achilios and Serenthia immediately attack Uldyssian when he returns from Kehjan, believing he is under control of the spirit of Malic. They do their best to kill Uldyssian but he overpowers them and Mendeln then shows up to ensure them that this truly is Uldyssian. It is at this point they realize the angel lied in order to manipulate them, and and is doing so all over the world; trying to create as much chaos as possible to ensure an easy victory for his army of angels. Mendeln then reveals to the others that the angel is most likely the angel of justice, Tyrael. Achilios then suggests an alliance with Inarius, desperate to at least ensure the survival of Sanctuary. The offer to join forces with Inarius is made by Rathma and results in Inarius throwing a tantrum and sending Rathma into the void.
The night before the inevitable battle between the Edyrem and the Cathedral, Achilios is, as always, scouting ahead. Something seems odd and he realizes the tall grass around him moves, yet there is no wind. Upon the discovery, the grass begins attacking him and after a struggle in which Achilios cuts the grass but it immediately grows back, the undead ends up being pulled underground by the grass. He remains trapped there the entire night. In the morning, Uldyssian is overcome by his emotions as the grass attacks his Edyrem. He responds by burning the grass and any nearby vegetation. Achilios is finally released from the earth when the very ground around him rises up. He sees Mendeln battling with Malic in the body of Jonas and quickly assists him. Malic tries to send his spirit into Achilios' body but Achilios is already dead, and so the attempt fails. Soon after, Mendeln stabs Malic with his bone dagger which releases Malics spirit from Jonas' body to be sent back to the afterlife. After stabbing Malic, Mendeln is knocked unconscious and Achilios, unsure of what to do, places the bone dagger on Mendeln's chest. Ghosts appear, ghosts of the very first Nephalem, and all of them are drawn into the dagger as Mendeln is revived. 
Malic disposed of, Achilios set his sights on assisting the Edyrem. Seeing Serenthia, he came to her aid and demanded she flee from the battle. Whilst arguing that she wouldn't leave him she was cut down by an angel, and died smiling. Later, after Uldyssian sacrifices himself for Sanctuary, Serenthia is alive, and Achilios is again very much alive. 
- Contrary to many other types of undead, the undead of Diablo are quite quick. Many times throughout the books it was mentioned that Achilios, in life, was agile, but after death he was much more agile and stronger as well.
- It is suggested that even magic users in the world of Diablo cannot sense an undead upon them, as Lilith never sensed Achilios coming. The only beings that can sense Achilios in the Sin War are those that had relationships with him in life, and incredibly powerful beings such as angels, the first Nephalem, and Trag'Oul.
- The dirt clings to the undead, as if it knows there is something not right with them:
Now and then, when he was forced to lean against a trunk, he left in his wake fragmented handprints of dirt. Soft, moist dirt. It seemed no matter how much he tried to wipe his hands clean, there was always more. 
- Undead do not sleep, or breath for that matter. They speak through some magic granted to them when being re-animated.
- Undead do not blink except when they consciously want to
- Achilios is always cold. The only times he feels the sensation of warmth is when he is near to the people he held dear in life.
- Achilios kept his magical abilities even in death.
- The undead have a phobia of being buried
- The undead talk slowly, (Mendeln speculates it may be because of dirt trapped in the body's windpipe):
"I have no... time... for you..." It was the first words he had spoken in days and the croaking sound of it repelled him as much as it seemed to the animal.
- Under the power of Tyrael, when facing the Thonos, Achilios' eyes blazed white, and he spoke like a living being.
- The Veiled Prophet, page 10. Richard A. Knaak, 2007.
- Scales of the Serpent, page 94. Richard A. Knaak, 2007.
- The Veiled Prophet, page 107. Richard A. Knaak, 2007.
- The Veiled Prophet, page 310. Richard A. Knaak, 2007.
- The Veiled Prophet, page 325. Richard A. Knaak, 2007.
- Scales of the Serpent, page 60. Richard A. Knaak, 2007.
- Scales of the Serpent, page 61. Richard A. Knaak, 2007.
|Characters in the Diablo novels|
|The Sin War Trilogy||Humans||Uldyssian • Mendeln • Serenthia • Achilios • Lylia • Malic • Rathma • Bul-Kathos|
|Angels||Inarius • Imperius • Auriel • Itherael • Malthael • Tyrael|
|Demons||Lilith • Lucion • Astrogha • Gulag • Mephisto • Diablo|
|Legacy of Blood||Norrec Vizharan • Sadun Tryst • Fautzin • Kara Nightshadow • Hanos Jeronnan • Augustus Malevolyn • Galeona • Meshif • Drognan • Horazon • Xazax|
|The Black Road||Darrik Lang • Buyard Cholik • Kabraxis|
|The Kingdom of Shadow||Kentril Dumon • Quov Tsin • Juris Khan • Zayl • Humbart Wessel • Diablo|
|Moon of the Spider||Zayl • Humbart Wessel • Aldric Jitan • Astrogha • Karybdus|
|Demonsbane||Siggard • Assur • Tyrael|