Legacy of Kain is a pentalogy of action/adventure video games revolving around the "life" of the vampire Kain and his rebellious scion, Raziel. The first and fourth games center on Kain, while the second and third center on Raziel, culminating with both stories coming together in the fifth game.
The sixth game, Nosgoth, was a pvp game that did not include Kain. It was never officially released, but began open beta in January 2015. The game officially shut down its servers on May 31, 2016. It seems to have had little effect on fanfiction.
The games take place in the world of Nosgoth, where two powerful races, the Vampires and the Hylden, have been at war for centuries. At the time of the games, the Hylden have been banished and kept out of the world by the nine Pillars of Nosgoth and their respective guardians, but the guardians have become corrupt and insane, and the Hylden seek to overthrow them entirely.
Kain, a nobleman murdered by assassins and brought back as a Vampire by a necromancer, is meant to restore the Pillars in order to break his curse, and then assume his role as the next Guardian of Balance. Instead, shenanigans involving time travel ensue, and Kain brings about the near-extinction of the Vampires and, in the end, chooses the continued existence of his race over sacrificing himself to restore Nosgoth.
After building his empire in the corrupt Nosgoth, Kain raises six lieutenants from fallen warrior-priests known as the Sarafan. His first lieutenant, Raziel, inexplicably evolves into a winged form, surpassing Kain. Seemingly out of jealousy, Kain rips the wings from Raziel's back and has him thrown off a cliff into a huge whirlpool called the Lake of the Dead, but Raziel survives as a soul reaver, who must consume the souls of the dead to survive. He infiltrates Kain's stronghold and confronts him. The two battle, during which time Raziel is struck by Kain's sword, called the Soul Reaver and sharing the ability to consume souls. The paradox of the two coming in contact shatters the sword, and its spirit attaches itself to Raziel as a symbiotic spectral weapon. Kain escapes, and Raziel pursues him, killing all of his brothers but one in the process. They confront each other again in the Timestreaming Chamber of Moebius, the former Guardian of Time, and Raziel follows Kain through the device, leading them into Nosgoth's past.
More time travel shenanigans ensue, causing alternate timelines and slowly unraveling the true destiny of Kain, Raziel, and the Soul Reaver, which are inextricably entwined with the fate of Nosgoth.
The LoK fandom is small, or at least not very obvious. Goodfics tend to be very good, both the serious and the ridiculous. Badfics tend to be very bad, while the crackfics tend to be very, very strange. Many stories fall into the middle range of being readable and not very damaging to canon.
Because of the odd canon names, minis abound.
Popular plots include the six lieutenants as fledgelings, stories set during the height of the empire, continuations of the games, or the lives of secondary characters. Popular OC plots include portal fics, romance, Seventh Lieutenants, and vampires adopting human children. Scribbles, drabbles, and songfics are also popular.
Kain/Umah stories are rarer than Canon/OC. The most popular ship is Kain/Raziel followed closely by Janos/Vorador, which are both vaguely incestuous as well as extremely improbable. Kain/Vorador, Janos/Raziel, and even Kain/Janos stories exist. Sex stories about the canon couple, Nupraptor and Ariel, seem to not exist, making it rarer than Moebius/Mortanious.