The Genius (典災, Ten-wazawai?, also known as the "Gatherers") are Monsters in the Elder Tale universe that did not exist prior to the Catastrophe. They are the main antagonists of the Log Horizon TRPG, hoping to spread death and destruction around the world. In actuality, they are foreigners to the world of Theldesia, and are one of the two Traveler factions.
The monster bodies themselves were supposed to be implemented with the Homesteading the Noosphere expansion pack; however, eight hours before they were supposed to appear, the seal on the Fortress of Apocalypse from which they would come suddenly broke (perhaps because of the Travelers), letting the monsters loose into the world.
There are over 90 Genius in the world, but only some of them are active. However, those that are active caused the disappearance of almost all the Ancients and the Thirteen Global Chivalric Orders. Their abilities are above the scope of the world's laws, and they hold knowledge about the Adventurers' old world. The Geniuses' powers hold the secret to this world, and must be defended from by the Adventurers.
In fact, the Genius are a faction of the Traveler race, that was brought to this world during the Catastrophe, which they called the Eclipse, and their efforts are on bringing destruction and chaos. The Genius specialize in gathering Empathiom, and they tend to justify any means to achieve their aims. Additionally, their monster avatars’ destructive flavor texts are only exacerbating the situation.
Over time, the Genius have demonstrated the ability to use the Elder Tales game systems to wreak havoc. For example, Bucaphi gave Krusty a hitherto unseen status effect that blocked all forms of HP recovery, blocked telepathy, prevented him from moving across server boundaries, and stripped him of certain memories. Papus hinted that Colopatiron can summon high-level, raid-ranked monsters in areas where they wouldn't normally have appeared when Elder Tales was a game. Finally, during its invasion of Akiba, Eirenus exploits the Teacher System to forcibly lower the levels of all the veteran adventurers in its dungeon to Level 35, and said Adventurers are stuck with this level even when killed and revived in the regular Akiba's Cathedral.
"Genius" seems to be a subclass, although perhaps restricted only to monsters. All of the known Genius have this subclass.
Route 43 arc
Roe2 mentions the Genius monsters when talking to Minori during the invasion of Wyverns, saying that the Odysseia Knights' repeated suicides may be part of the monsters' plans, since it creates something called "Empathium."
Kanami, Go East! arc
In China, Kanami's Party hears reports that the Lelang Wolf Cavalry was wiped out by undying monsters. Upon hearing this, Kanami decides to go face down these monsters, with Elias Hackblade and Coppelia following her. Although Leonardo initially hesitates, he agrees to go after talking with K.R.. The party is successful in killing the two Genius monsters they face, although K.R. is killed and sent back to Yamato in the process.
While talking to Ga-tan, K.R. reveals that he joined Plant Hwyaden in order to gather more information about these monsters. From his research, the Genius were moving around the server and walking around, unseen and unknown. One is in Nakasu, while two are in Akiba.
Shiroe reads Roe2's letter, which was written while Touya's Party was in Sapphir. The preface explains that she is a Traveler, a race of mental bodies that were brought to the world of Theldesia because of the Catastrophe, which they called the Eclipse (its kanji reads "pact"), and continues to explain that her race has two factions: the Fools and the Genius. The kanji for Genius, reads "collectors," giving a hint into their true nature.