Hikaru Takabe is an anti-social young girl who wants nothing more than to be left alone. One night, while on a school trip, she goes to the beach when suddenly a meteor falls from the sky killing her instantly. The next thing she knows, Hikaru is sitting in class and believes the event from the beach to be nothing more than a daydream. However, when a mysterious voice begins talking directly into her head it informs her that she really did die that night and in order to revive her, the alien presence, known as Horizon, must always reside inside her. However, Horizon also has another purpose for coming to this world; after chasing a malevolent being known as Maelstrom across the universe, his target - which has the power to destroy all creation - sought sanctuary on Earth. Now Hikaru must help Horizon to find Maelstrom hidden amongst the 7 billion people on earth and kill him before it’s too late.
In secret, alien parasites drift downwards toward Earth. Their directive: to take control of a human body and thrive in secret. When a parasite attempts to take over Shinji, an ordinary high school student, he stops it in his arm to save his mind. With the strange power of amorphous muscle, the curious parasite strikes an uneasy truce with Shinji: it will keep him alive and strong so that it may continue living, and will help protect him from the other parasites that might not take kindly to Shinji's mind still actively working. Can Shinji gain the courage to face the parasites and protect humanity? And would it even make a difference if he did?
7 Billion Needles is a tough one for recommendations since each volume has a pretty different feel, but the first two vols specifically (especially with some of the dark comedy elements in volume 2) remind me a lot of a tamed down, more serious Parasyte due to similarities in plot (both overall, and certain plot points I won't go into for spoiler reasons). Parasyte is pretty ridiculous and over the top gory/comedic in some ways, while 7 Billion Needles is more serious - but still, fans of one will probably appreciate the other.
If you liked Parasyte, you might like 7 Billion Needles because both series feature a student merging with an alien to fight other aliens.
Recommended if you're into the creepy human/alien symbiosis/body-sharing in either manga. Both also have some fairly graphic imagery, and stories that morph and change so rapidly that each volume is very different from those before/after it.
In the idyllic, suburban Soil New Town, everyone's lives are picturesque, down to the mandatory perfectly-manicured flower bed in front of every house. But this peaceful façade is shattered when the well-known Suzushiro family disappears during a power outage, leaving behind nothing but a pillar of salt. Things get even weirder with the arrival of a mountain of this same salt at the local school, a crossdresser fixated on transmission towers, and threatening messages demanding that every citizen pay a portion of the Suzushiro family’s ransom. The plain and often-frazzled detective Onoda and her sexist and hygienically-challenged boss, Sergeant Yokoi, are called in from out of town to help, but it's a tough case to crack. Will they be able to get any information out of the gossiping townspeople, all of whom hate outsiders and have something to hide?
Soil is way better than 7 Billion Needles, but they still remind me of each other a bit (especially the second volume of 7BN).
Both of these dark seinen manga have supernatural elements that invade a normal setting and wreak havoc, and fairly horrific imagery. Will probably appeal to the same audience.