Continuing on from the last arc, Rias and Asia are both living at Issei's house, and Akeno is trying to help Issei regain control over his left arm from Ddraig. Issei has finally found work with a reliable client, a young man with black and blonde hair and similar interests to him. One day while the group is at Issei's house for club activities, Kiba is perturbed by a childhood photo of Issei and a young boy that features a Holy Sword in the background. Kiba begins to act strangely, no longer helping in battle. Rias reveals to Issei that Kiba was once part of an experiment named the "Holy Sword Project", of which he was the Sole Survivor. He managed to escape, and was found on the brink of death by Rias. Meanwhile, Kiba finds Freed Sellzen killing a priest with a fragment of Excalibur. Enraged, Kiba attacks him, but Freed escapes.
The next day, the group are visited at Issei's home by two girls from the church: the blue-haired Xenovia Quarta and Irina Shidou (whom Issei recognises as his childhood friend from the photo he had mistakenly thought was male). Issei later goes fishing with his client. Xenovia and Irina say that they are looking to hunt down the scattered fragments of Excalibur before Xenovia attempts to kill Asia, whom she recognised as the "Holy Maiden", for becoming a Devil. Angered, Issei and Kiba decide it's on and they engage in a duel. They lose to the girls, who explain to Rias that the Excalibur pieces were stolen by Kokabiel, one of the leaders of the Fallen Angels, before departing.
Kiba is upset over his loss. Noticing this, Issei recruits Saji (Sona's [Pawn]) and Koneko to help him track down Xenovia and Irina. When they do, they negotiate with the girls to help them retrieve the Excalibur swords and destroy them. The five meet with Kiba, who was touched by Issei and Koneko's actions, that night to put their plan into action. Being busy, Issei cancels on his client, who sits on the pier alone before being joined by a young, silver-haired boy around Issei's age.
Issei, Kiba, Koneko, Saji, Xenovia and Irina are on the hunt for Excalibur fragments. They track down Freed at an abandoned warehouse, and they fight him. They have the advantage until a man named Valper Galilei aids Freed in controlling his sword. Freed manages to escape, and is chased after by Xenovia, Irina and Kiba, while the others stay behind and get punished by their respective club presidents. When the group find an injured Irina in the outskirts of town, they are confronted by Kokabiel himself. Kokabiel reveals that his intention is to create another Great War between the three factions. He plans to attack the devils of Kuoh Academy so the Satans will be forced to take action.
Kokabiel, Valper and Freed arrive at Kuoh Academy. The Student Council form a barrier around the school as the battle begins. Kokabiel summons several Cerebus demons, which the club take out with Xenovia's help. Valper combines the four Excalibur fragments, triggering a twenty-minute countdown on a self-destructing magic circle. Avenging the children that died in the Holy Sword Project, Kiba reaches Balance Breaker and destroys the Fused Excalibur. Kiba turns on Valper, but Kokabiel strikes him down instead.
Issei transfers his powers to Rias, but it's no use. Kokabiel mocks Akeno for being the daughter of the Fallen Angel general Baraqiel, angering Issei and Akeno. Kiba, Koneko and Xenovia distract Kokabiel while Issei attacks, but he still manages to overpower all four of them. Seeing Issei protect her, Akeno realises that she really does have romantic feelings for him. At this point, Kokabiel shatters their psyches by revealing the horrible truth behind the Great War's outcome: "In the Great War, the Four Satans died, and so did God."
While the shock of this breaks the group's spirits, Issei prepares to take on Kokabiel once more when the barrier is broken down. Entering the field is a new figure - the Emperor of the White Dragon, Albion. He single-handedly defeats Kokabiel and negates the self-destructive power of the magic circle before threatening Issei and leaving with Freed and Kokabiel. Sona and her team stay behind to fix the damage done to the school building, while Kiba comes to terms with his past and vows not to let Rias down again. The next day, Xenovia has joined Kuoh Academy and the Occult Research Club as Rias' [Knight] after asking Rias to reincarnate her after hearing the truth about God's demise. She apologises to Asia for her earlier remarks and is Easily Forgiven. Later, as Issei plays a video game with his client, an avid collector. When the man calls Issei "the Emperor of the Red Dragon", Issei soon discovers that the latter is actually Azazel, the leader of the Fallen Angels.
Volume 3 of High School DxD contains particular-to-this-volume examples of the following tropes:
- Adaptation Expansion: The anime added in the subplot of Issei's new client turning out to be Azazel. In the original light novels, Azazel only appeared from Volume 4 onwards.
- All for Nothing: After several years of leading the Fallen Angels, Kokabiel's attempt to start another Great War goes up in flames: the three of the seven Excaliburs he stole are retrieved and sent back to Rome, his plot to kill the Gremory family have been foiled and on top of that, he ends up being overpowered by Vali and captured by Azazel to be frozen in Cocytus.
- Anti-Climax: The huge, two-episode-long fight between Kokabiel and the Occult Research Club seems to be coming to a climax as Issei rouses his friends' fighting spirits and prepares for one last attack, when a new character, Vali, abruptly enters the battlefield, subdues Kokabiel and leaves with him.
- Arc Villain: Kokabiel's plans to restart the Great War with the use of the Holy Sword Excalibur drives the volume's plot and stopping it are the Occult Research Club and the exorcists' priorities.
- Bad Guys Do the Dirty Work: The one who ends up killing Valper is Kokabiel, who is in turn defeated and captured by Vali and Azazel.
- Beyond the Impossible: Kiba's newfound ability to create Holy Devil Swords is something that shouldn't have been possible, according to Kokabiel.
- Bittersweet Ending: Kokabiel has been defeated, Kiba comes to terms with his past, the Excalibur swords are recovered and Xenovia joins the Occult Research Club. However, Xenovia is exiled from the Church after learning that God Is Dead, Irina doesn't know about what happened to God, Albion and Vali escape, and Azazel reveals himself to be a fallen angel.
- Both Sides Have a Point: Xenovia and Irina are followers of God who don't want to risk getting devils involved. A reasonable point since the Church often exorcises Devils and Fallen Angels. Kiba wants to stop Kokabiel's plans to retrieve the Holy Swords on the equally fair point that his friends were cruelly experimented on and were killed shortly afterwards, save for Kiba.
- Break Her Heart to Save Her: A platonic version occurs when Xenovia leaves the church with little explanation spare Irina, who is an especially devout believer, from telling the truth about her exile and transformation.
- Broken Pedestal: Xenovia loses the church's trust after she learns the truth about God's death.
- Call-Back: When Xenovia meets Asia, she calls Asia a "witch", which is what the Church referred to Asia as way back in Volume 1. Ironically, Xenovia herself becomes a heretic towards the end of the volume.
- Chekhov's Gunman: Azazel, the young man who meets with Issei's client on the pier ends up showing up at the end of the volume to defeat the main antagonist Kokabiel.
- An enraged Kiba unlocks the Balance Breaker form of his Sacred Gear "Sword Birth", and performs the impossible act of creating a holy sword out of demonic energy. This confirms some of the older characters' suspicions that both God and Satan are long dead, and the laws of reality are no longer operating as intended. In fact, Sacred Gears (originally created as a way for God to empower human champions) were never intended to be Evolving Weapons at all, and the ability of some of them to kill gods is likewise a relatively new development.
- Cruel and Unusual Death: The children involved in the Holy Sword Project were brutally subjected to harsh experiments and eventually killed after their attempts failed.
- Curb-Stomp Battle:
- Kiba and Issei are easily defeated by Xenovia and Irina in their mock duel.
- The Occult Research Club and the Student Council helps beat up Freed and Kokabiel's demons.
- Vali easily defeats Kokabiel towards the end of the volume after the latter overpowers Issei and his allies.
- A Day in the Limelight: Yuto Kiba gets a prominent role in this volume, focusing on his involvement with the Holy Swords.
- Death Glare:
- Issei to Xenovia and Irina when Xenovia is about to kill Asia.
- Xenovia and Kiba when the former attempts to kill him as he's a stray devil.
- Easily Condemned: As soon as Xenovia learns the truth about what happened to God in the Bible, the Church turns its back on her and banishes her as a heretic.
- Easily Forgiven: Asia forgives Xenovia at the end of this volume for her earlier actions.
- Enemy Mine: Issei goes to great lengths to negotiate with Xenovia and Irina to retrieve the stolen Excalibur fragments and get Kiba off of his Roaring Rampage of Revenge.
- God Is Dead: This arc reveals that God was killed alongside the original Four Satans in the Great War, and Archangel Michael has been running Heaven ever since.
- Groin Attack: During the dodgeball game, Issei has to take a ball directed at Kiba, who'd been out of it since he saw a Holy Sword in Issei's childhood pictures. The ball happens to swerve down and strike him where it hurts. Rias initially doesn't get which "balls" he's trying to tell her about until Akeno clarifies things, whereupon she asks Asia to heal Issei's crotch before she loses the chance to make good use of it. Issei is reluctant to expose his genitals to Asia, so he talks her into using her Healing Hands over her clothes.
- Happy Ending Override: For the first story arc. So, Asia has been reincarnated as a devil, Raynare is killed, Issei nullifies Riser's engagement to Rias and peace has been restored, right? It turns out Kokabiel is searching for Excalibur and is planning to start another great war between devils, angels and fallen angels.
- Heel–Face Turn: Xenovia goes from a Church exorcist to a member of the Occult Research Club after learning the truth about God.
- The Heretic:
- Valper was declared a heretic because of his involvement in the Holy Sword Project.
- At the end of the volume, Xenovia ironically becomes a heretic for understandably learning about God's death from Kokabiel. As such, she has Rias transform her into a devil.
- Heroic BSoD: Everyone's reaction to Kokabiel revealing that God has been dead since the Great War, especially for Xenovia and Asia. It even inspires Xenovia to join Rias's peerage as a devil, since she has burned her bridges with the Church.
- History Repeats: Once again, a church official is exiled as a heretic and is transformed into a devil to join the Occult Research Club.
- Impaled with Extreme Prejudice: Kokabiel runs Valper Galilei through with a spear in volume 3.
- Irony: Asia was excommunicated from the Church as a heretic after healing the devil Diodora in her backstory. At the end of this volume, Xenovia herself gets branded as a heretic, this time for learning the truth about God. Not only that, but Xenovia has Rias transform her into a devil, who are enemies of the Church.
- It's Personal: Kiba wants to avenge the children involved in the Holy Sword Project.
- Laser-Guided Karma: By the end of this volume, Xenovia learns from Kokabiel that God Is Dead. This leads to the Church exiling her as a heretic shortly after Kokabiel is sealed for good.
- Locked Out of the Loop: Xenovia decides against telling Irina the truth about God as her faith is stronger than hers and the truth could've caused irreparable mental anguish.
- Make an Example of Them: Xenovia attempts to kill both Asia and Kiba as a reminder that the Church is willing to exorcise devils whenever the situation calls for it. In both cases, Issei intervenes.
- Mass "Oh, Crap!": The Occult Research Club has a collective moment of terror when they realize that God Is Dead all along.
- Moment Killer: In the first episode of NEW, Rias tries to seduce Issei but Asia knocks on the door, underscored by a Sudden Soundtrack Stop.
- Nice Job Fixing It, Villain: Sure, Kokabiel telling the truth about God's death was a shocker to the Occult Research Club, but it did contribute to Xenovia's transformation into a devil and joining the Occult Research Club in the long run.
- Non-Protagonist Resolver: Kokabiel isn't beaten by any of the main protagonists; that honor instead belongs to Azazel and Vali Lucifer.
- Oh, Crap!:
- Asia when she realizes that Xenovia's going to kill her during their first meeting.
- Rias, Akeno and Koneko's reaction when Issei is about to call Xenovia out for her actions towards Asia (in the anime at least).
- Pet the Dog: Once the battle with Kokabiel is over, Xenovia gives a sincere apology to Asia for her earlier actions.
- Prepare to Die: Twice by Xenovia. The first time happens when she decides to kill Asia upon realizing that she has been faithful to God even after becoming a devil. The second time happens when Xenovia learns that Kiba became a stray devil after leaving the Gremory family. Fortunately, Issei intervenes in both instances.
- Put on a Bus: At the end of this volume, Irina leaves for Rome with the Excalibur fragments and isn't seen again for the next three volumes.
- Relative Button: Xenovia prepares to kill Asia when she tells her she still believes in God. Issei, seeing what is happening, can't stand to see Asia being killed and challenges Xenovia to a duel.
- The Reveal: Two major ones, both delivered in the form of the Wham Lines below.
- Screw This, I'm Outta Here: Kiba leaves the Occult Research Club to pursue his Roaring Rampage of Revenge on Galilei because of his involvement in the Holy Sword Project.
- She Knows Too Much: After Xenovia learns that God Is Dead, the Church has no choice but to exile her as a heretic.
- Shoot the Dog: Xenovia attempts to kill both Asia and Kiba on the grounds of the former being a heretic and the latter a stray devil. In both cases, Issei disagrees.
- The Stinger: The final episode of the arc has a post-credits scene revealing Issei's client to be Azazel, the leader of the Fallen Angels.
- Sudden Soundtrack Stop: Rias tries to seduce Issei once again in the first episode of NEW and then Asia knocks on the door [insert quick music slowdown] and tells Issei that they're going to start their training.
- This Means War!: Issei declares war on Xenovia and Irina after they taunt and attempt to kill Asia for becoming a devil.
- Well-Intentioned Extremist: Xenovia is willing to kill devils as an exorcist to uphold the Church's ideals, even those who still believe in God. Well, at least until the aftermath of Kokabiel's battle.
- We Will Meet Again: Vali tells Issei that they will meet up again to fight before he leaves.
- Wham Line:
- One of the series' biggest reveals, courtesy of Kokabiel:
Kokabiel: In the Great War, the Four Satans died, and so did God.- Another one comes in the form of The Stinger:
Issei's client: How about another match, Devil-kun, or should I say, "Emperor of the Red Dragon"?
(Issei gasps)
Issei's client: I am Azazel, leader of the Fallen Angels. - What the Hell, Hero?:
- Issei points out to Xenovia that Asia was alone, she's a kind person and is also her friend and family. He also notes during his This Means War! declaration that if Xenovia ever attempts to kill Asia, he will consider her his enemy.
- Irina also calls Xenovia out for deciding to become a devil during the volume's finale.
- Yin-Yang Bomb: Kiba's new Balance Breaker lets him infuse his created swords with both Holy and Devil energy.
- You Can't Go Home Again: After learning the truth about God and being declared as a heretic, she can't return to the Church like Asia.
- You Can't Thwart Stage One: Kokabiel created the Fused Excalibur... but the heroes foil his plans just in time.
- You Have Outlived Your Usefulness: After the Fused Excalibur is created and then shattered, Kokabiel kills Valper, his subordinate.
