OK, time to look at what makes a manga: it's villains.
First, we have Crona and the Demon Sword, Ragnarok.
Let me explain what the Black Blood is: the Black Blood is like normal blood, only black....if normal blood drove people who had it TOTALLY BATSHIT INSANE.
Ragnarok is a jerk. He has a symbiotic relationship with Crona (he is where the black blood comes from) and, when summoned.....bursts out of Crona's back. Euurrghhhhh.....squick, indeed. Even worse? Ragnarok can WEAPONIZE Crona's black blood, turning it into needles, spikes, flying razors, etc. Yikes.
The one villain that scares me the most is Cthulhu- uhm.. I mean, Asura!!!!
And if the picture isn't enough to make you need to change your pants, read Wikipedia's description of him:
The first kishin or demon god, and the ultimate antagonist in the series. He is known for his exceedingly paranoid disposition and quirky habits such as wrapping his entire head with countless scarfs and wearing five or six layers of clothes. He was fearful of everything, including his weapon partner Vajra, whom he devoured whole, though he can still cough up Vajra to attack. The scarfs on his body are actually made from his own overstretched skin, and can also be used as weapons. As the embodiment of madness, Asura emits a wavelength that causes people to lose their grip on reality and hallucinate. He was one of Shingami's strongest subordinates 800 years prior to the start of the series. Out of fear he began devouring innocent souls, including his own weapon partner; Lord Death ripped off his skin and used it as a bag to imprison him beneath Death City. He is eventually released thanks to Medusa and her minions, and goes off into hiding to spread his madness across the world. He is rarely seen in the manga though he does become the center of focus for the anime, particularly Arachne's plans to spread his madness across the world.
is Medusa's older sister, a witch with a spider motif who loves to mess with your mind....