Artifact Title: Shin doesn't wear a mask, he physically transforms into Kamen Rider, nor does he ride his bike (a Suzuki Bandi 250) in his transformed state.
Badass: Sarah Fukumachi, the CIA operative. Over the course of the film she orders a raid on Onizuka's lab (and is the only survivor), saves Shin from a sniping attempt, bursts in on the Nebulous Evil Organization's transport of the transformed Onizuka with a rocket launcher, and most likely would have killed Shin in the climax had an organization helicopter not gotten in the way.
Bloodier and Gorier: Shin's "finishing move" is to cut a monster's head and pull it off complete with part of the spine and dripping neon green blood (Eat your heart out, Sub-Zero.) Yes, there actually is a Rider with a more violent attack style than Kamen Rider Amazon.
Curb-Stomp Battle: Shin killing the Big Bad who just killed his Love Interest. The former is a grasshopper-monster complete with super-strength, sharp claws and ultraregeneration powers, the latter is a normal human with a gun...yeah.
Cut Short: Despite the fact that the title calls it a prologue, Shin never went any further than the one movie. Kamen Rider Decade's comedy net spinoff has Shin asking the Decade cast to help his petition for a sequel, and subsequently finding success and fame only for the whole thing to be All Just a Dream. Poor Shin.
Darker and Edgier: No shit. This film is the darkest and grittiest interpretation of Kamen Rider to date. Rather than a tranditional super-suited human, the Rider is genetically altered and undergoes a horrifying transformation. The fight scenes are visceral and bloody. And if that weren't bad enough, almost everybody in the film dies. Considering that the Kamen Rider fanbase is normally very supportive of darker installments (Kamen Rider Black, Kamen Rider Faiz, etc.), the controversial nature of this film amongst the fandom is quite the indicator of what this film is like.
Kill 'em All: By the end of the movie, pretty much every named character except Shin and his gym buddy, who was nowhere near the scene of carnage, is dead. Including Shin's girlfriend, who was pregnant with his child.
Took a Level in Badass: Shin doesn't really do this in his own movie, but in the anniversary movies for Decade and OOO, where he fights alongside the other Riders and even gets his own bike, and in All Kamen Rider Generation, where he acts more like a real hero.
What Could Have Been: Had the film been successful, there would've been other movies following Shin who would "evolve" into a more Kamen Rider form (A belt and armor)