Urotsukidouji is more about the ultraviolence. The manga was about the plot and sex. They totally warped the manga when they made the anime. The manga involved tentacled lizards having sex with girls (because tentacles don't have to be censored, true story). The guy that made the anime stated nothing stirs up people more than sex and violence, so he got the bright idea to throw them in together and add creepy demons; yes, some of the demons in Urotsukidouji are fucking scary -- the high-pitced voice one (in the dub) that wobbles his head creeped me out so bad for years.
Amano-Jaku, one of the Man-Beasts, is sent to the human realm on the pretense to search for the Overfiend, a legendary being rumored to unite the three realms (demons being the third), but really he's being punished for who his girlfriend is. Amano-Jaku's sister Megumi goes to the human world as well and they make a bet on who will find the Overfiend first. Amano has his eye on a star college athlete named Ozaki - physically fit, handsome and all the women love him (he can sleep with four women at once, he's that hot). Ozaki catches a dorky boy named Nagumo peeping on the cheerleaders and nails him with a basketball, cutting Nagumo's face. He licks the blood off Nagumo's face and apologizes while everyone laughs at Nagumo. Later that night during Ozaki's sexcapapes, Amano-Jaku watches as demons attack him, since they also think Ozaki possibly the Overfiend (most humans don't know about the legend of the Overfiend, though). This scene is fucking scary as Hell! A demon swallows Ozaki whole, then Ozaki transforms into a werewolf-like creature and slaughters all the demons. Amano rejoices but decides to test Ozaki and attacks him. To Amano's horror, Ozaki can't withstand the attack and dies. He then realizes it wasn't Ozaki who was the Overfiend, it was Nagumo and it was Nagumo's blood that made Ozaki turn into a fiend.
Meanwhile Megumi, who is a total slut, has her eye on Nagumo (something didn't smell right about him and she figured the Overfiend would be a sex fiend). She decides to test Nagumo and have a little fun with him, so while he's on a date with the cheerleader he was peeping on (she was attacked by a demon and thinks he saved her, even though it was really Amano who saved her), Megumi pounces on him and tries to bang him. Shocked, Nagumo scrambles away towards the street and gets hit by an oncoming truck. He's declared dead at the hospital, leaving Megumi and Amano scratching their heads. Then miraclously Nagumo comes back to life, turns into a HUGE demon and destroys the hospital. Akemi, the cheerleader, is glad Nagumo is alive, but a fellow student, Nikki who also has a crush on Akemi, isn't happy. The demons find him and use his jealousy to ensnare Nagumo in a fiendish trap. Nikki abducts Akemi and uses her to lure Nagumo out to a construction site, where he crushes Nagumo under steel girders. Nagumo comes back to life and attacks Nikki with his blood. Nikki eats some of Nagumo's blood and gains some of Nagumo's power, but Nagumo manages to defeat him. Megumi chases off the demons while Amano confronts a powerful demon who explains to him why the demons are deadset on stopping the Overfiend (they like the current balance of the universe and believe the Overfiend will actually destroy the world).
Meanwhile Akemi tries to console Nagumo, who's upset at what he's become and about killing Nikki, who used to be his friend. The young lovebirds get passionate and have sex (it's hentai, so it's natural). During their lovemaking Nagumo once again turns into a fiend and impregnates Akemi. A mysterious power transports her to Osaka Castle and Nagumo grows into a monstrous fiend that starts destroying the city around him. The portals between the Man-Beast realm and Demon realm and human realm are torn assumnder as Man-Beasts and Demons rush into the human realm. Some try to attack Nagumo and are wiped out, while others go around just having sex (again, it's hentai). Amano and Megumi are spared because they introduced themselves to Nagumo and Akemi. Amano is tasked with beholding the birth of the Overfiend, meaning Nagumo isn't the Overfiend. Amano learns Nagumo's role was to prepare the world for the coming of the Overfiend.
The next movie was a sequel but took place during the first one. It introduced Muni Hausen II, the son of a mad scientist and occultist in Nazi German who tried to summon the Demon King (not to be confused with the Demon Lord) to counter the Overfiend.
The SS had Muni Hausen executed when the ritual failed and Muni Hausen II lost his arm in an accident during the ritual. The boy was ridiculed and abused in his boarding school, so he used his teachers as offerings for a ritual to summon the Demon Lord (who had an affair with Megumi and got on the Demon Queen's bad side). Through the Demon Lord Muni Hausen II learns about summoning the Demon King.
Years later Nagumo's relatives fly in to Osaka to visit, but demons under Muni Hausen II's guidance attack the plane and only Nagumo's cousin Takeaki survives. Nagumo offers his blood for an emergency blood transfusion. Muni Hausen II torments Takeaki with demon whores to force Nagumo's blood to awaken in him. Using the demonoid Takeaki, Muni Hausen II has Takeaki abduct women to use as offerings in his own Demon King Summoning ritual. Takeaki realizes it's him doing all the kills and freaks out. Nagumo and Akemi worry about him. Megumi falls in love with him but the Demon Lord abducts Megumi to use her as the keystone of the ritual. Amanok-Jaku confronts Muni Hausen II and the Demon Lord while Nagumo tries to save Megumi and Takeaki. Megumi breaks free just before Takeaki (in demon form) kills Nagumo and saves Nagumo by killing Takeaki. While Megumi cradles the dying Takeaki in her arms, Amano defeats Muni Hausen II by hurling a helicopter rotor at him.
The third movie I never watched much and don't remember well. It introduces the Makemono race (demon beasts), newly spawned in the aftermath of Nagumo's destruction.
The Kanto (Tokyo) area is ruled by a dictator named Caesar, who's really a cyborg. His "daughter" is an android he created for sexual purposes (did you forget this was a hentai?). A surviving, rebuilt Muni Hausen II serves as his advisor and the two madmen try to summon the Demon King, which they kinda succeed at. When Muni Hausen II tries to attack Nagumo and Akemi, Amano-Jaku, a demon-beast named Buju, and Caesar's daughter try to stop him. Caesar is assassinated in the conflict, but his daughter dies. Nagumo remakrs on the irony of an android being so human.
The fourth movie follows Buju and his surviving comrades en route to Osaka when they encounter a city run by children. The children have psychic powers (part of the new world) and adults are used only for procreation before being discarded brutally. A couple of teens struggling to avoid being executed as adults ask Buju to save them. Buju defeats the ruling brothers and saves the teens.
The fifth movie was scrapped, but was supposed to show that the Overfiend wasn't the real Overfiend, but part of a new race of hermaphrodites. The real Overfiend was going to use this new race of hermaphrodites to replace all the previous races (human, demon, man-beast, demon-beast) and create a new universe. This got scrapped, as I said.