This page contains unmarked spoilers. You Have Been Warned!
High School D×D tackles some very serious subject matters even for a light novel series, in addition to touching on aspects on fanservice, harem and adventure... it isn't a series about characters from almost all religions and mythologies fighting each other for nothing...
- Issei's Death by Origin Story was bad enough, but later on, he desperately turns to run from Dohnaseek, get stabbed from behind, and fall to the ground bleeding out before Rias and (in the anime) her allies rescues him. Someone's having a bad day.
- Those with gynophobia (a fear of women) will be caught completely off-guard by the many times any female villain hunts down Issei.
- During a job in Volume 1, Issei comes across the client's room, before stepping in something wet, thick, and red - then the camera pans to show the trail of blood, before Issei completely and utterly freaks out. We don't see the entire body, but it's so bad that Issei is visibly struggling not to puke... and then as if to make the introduction worse, Freed shows us how happy he is to have done this.
- What's worse is that when Asia stumbles on the scene, is horrified and wonders how a self-proclaimed servant of God could be that needlessly cruel, he makes clear his intent to kill her, except that would offend his boss, so he settles for threatening her with Attempted Rape, if she doesn't shut up instead.
- While Freed was doing all that, Rias saw something that made her order a tactical retreat, forcing a grievously wounded Issei to leave Asia. We are never shown what it was that had Rias turn tail, but we can be sure it wasn't Raynare and her bunch because she easily vaporizes them later.
- Riser's beating up of Issei in volume 2 to twist Rias into ending their Rating Game can catch people off-guard. By the end of volume 2, Issei leaves serious burn marks on his face in their Final Battle.
- Issei may be a silly fellow half the time and is unquestionably the main protagonist, but even he can be pretty horrifying when he gets angry. Case in point:
I, who shall awaken; I am the Heavenly Dragon who stole the principle of domination from God; I mock the infinite, and fret over the dream; I shall become the Red Dragon of Domination; I shall drown you in the depths of crimson purgatory. Juggernaut Drive!
- One of the worst was Juggernaut Drive. Issei goes into an insane berserker state, during which he chews off Shalba's arms. He also grows a multitude of limbs from all over his body. And the real kicker? Ddraig has to warn Rias and the others to run away, because Issei would have killed them as well if they hadn't. The only reason this isn't High Octane is the way he was brought out of it. And this was only from Kiba and the rest's point of view. From Issei's point of view himself? It's a lot worse. Even though he is defeated, Shalba's look of utter terror as Issei attacks him is suitably unsettling.
- The second time he almost pulled off Juggernaut Drive was when he got himself knocked unconscious by Sairaorg in their match. He then wakes up to a white world with the past Boosted Gear users, surrounded by dark aura. One by one, they tell him to pull off Juggernaut Drive to the point he joins with them.
- The anime's version takes this Up to Eleven and can catch viewers completely off-guard. Issei's body grows draconic, slowly and painfully, while the form speaks in a voice somewhere between human and dragon, voices of past users whispering creepily all throughout. The resulting rampage is punctured by repeated cries of pain, mainly draconic but sometimes with Issei's screams overlaying it. Plus, Issei's Juggernaut Drive has the powers of Vali's Divine Dividing (stolen in their previous fight), and is able to No-Sell Gasper's Forbidden Balor View. Yikes!
- Picturing the demise of the other nuns at Diodora's hands. They met Diodora at his most affable. But he would soon brand them heretics, thereby exiling them from the church, and then they would rape and enslave them, as no one else would have anything to do with them. They would know that this devil is not just crazy, but a murderer. And it's too late to escape...
- Xenovia takes the chance to perform an exorcism on Asia and Kiba, making it a point that the Church opposes Devils.
- Shalba tries to kill Asia by sending her to the Dimensional Gap without hesitation.
- Rias being corrupted by Loki's clone of Issei at the end of BorN.
- Issei became this close to dying permanently at the hands of Shalba.
- The aftermath of Kokabiel and Katerea's attacks on Kuoh Academy. Witnessing part of the academy's destruction can be unsettling.
- Kiba's tragic past is unnerving. All of his friends dying by being subjected to harsh experiments just so they can hold a holy sword while praying to God not knowing that God Is Dead in this series.
- Shuri Himejima's death at the hands of her own relatives, given their resentment of Fallen Angels. To make matters even worse, her daughter Akeno was Forced to Watch her die. It foreshadows that Akeno's relatives were not about to let her escape alive, at least until she met Rias.
- Evil dragons in general can catch anyone off-guard, even those with dracophobia (fear of dragons). Considered by everyone as the most troublesome existences ever because they just love to destroy For the Evulz. Guess they got some lessons from Acnologia and Nefarian.
- The aftermath of the Vampire Civil War and Rizevim's assault on the vampire's homeland.
- Gasper losing it when told that Issei's dead and calmly killing Marius Tepes will also catch people off-guard.
- The murder of Cleria Belial at the hands of the devils for her illicit romance with Masaomi Yaegaki.
- Rizevim Livan Lucifer's demise. He might be an abusive grandfather and a malicious devil, but being curb-stomped and then Eaten Alive isn't the best way to go. The look on his face is suitably unsettling.
- As of the end of Volume 20, Trihexa is awake.
- Everything about Trihexa can catch people off-guard. Even without Apophis and Azakha boosting their forces with mass-produced evil dragon clones through their stolen Holy Grail, this creature could curb-stomp gods and even its exposed cores have a healing factor so strong, just one of them could take Issei's Diabolous Dragon and Sirzechs's full power head-on, and heal right up again, like nothing happened.