You'd know a yandere novel series is good when it can last over 200k words. (500k words is about 250 chapters of What is a Yandere?) In point, this novel is one of the top voted "must-read-bible" of the SFACG site pertaining to yandere tags. Anyway, this series had already concluded it's main series, and is now embarking on a sequel on what happened to them after they got together.

Here's a rough plot summary:

A girl, in an effort to make a boy's attention span absolutely 100% on her all of the time, goes all out to do whatever she could in order to do so. Like brainwashing him, massacring any girls who goes near him, or killing off his entire family chain so that he could only have one person to have ties with (herself, as she's claimed she's his gf), killing off their eventual baby they had to gain back his one and only affection onto her, and so on and forth. Apparently her family was a ancient one, with special powers in their blood, allowing them superhuman feats, and their family heritage held powerful control within the country. Which is bad news for the MC. And you know he's f**ked when the plot dictates like that so.

Personally, I would tag two tags to this novel, Blood/Gore (and Mature due to the reverse-aheming) as the numerous ways the yandere mu*dered people to achieve her means (including another yandere!). Like tying a girl upside down in an abandoned house, naked whilst being tortured and forcing her to eat her own sh*t as substenance for days until she got uh, loony. This is one series definitely not for the light-hearted readers.