The Xyacti merit some description; they're the mortal enemy of the Tesseracti, an unknowable alien force which corrupts echoes - pocket dimensions - until they're basically nightmare dimensions. Rather than being actively malevolent, though, the Xyacti are curious and trying to understand the world around them - like Jason Sartin said, they're radioactive toddlers, damaging their environs without really understanding what they're doing. They spread through memes, which means that they have to be fought through contradicting information - although, to be honest, I never sat down and wrote out what they were like. You can get some clues as to what they're like from the below.

XYACTI VICTORIA:

The Tesseracti live in mortal fear of the Xyacti because of exactly what happened to the Victorian Station echo. When it was discovered, it was an analogue of Victorian England, including fog-shrouded streets, a dirty sky the color of sepia photographs, a skyline festooned with smokestacks, and a steam-powered monorail that hung some five hundred feet over the city. (You boarded it by taking an elevator up to the top of a building.) There wasn't much in the way of technology that the Tesseracti could burgle, and so the dimension was used as a third-tier recruiting source for disposable muscle. 

In 1982, contact was lost with two Tesseracti operatives, both Anglophiles who had decided that they wanted to honeymoon in pseudo-Victorian London. A casual expedition to bring them back resulted in the deaths - well, as "dead" as you can get in an Xyacti-invaded echo - of eight Tesseracti operatives and the abrupt cessation of echo travel for approximately six days as the upper echelons fought a panicky civil war over what to do next. When communications were re-established with the expedition, the Xyacti used an open channel to jump into the Iron Labyrinth, spreading as a concept though three different levels before it ran out of people to infect. 

The Tesseracti burned a number of favors with the NanoBaja sublimed dimension in order to find out what was going on. NanoBaja, having already sealed themselves off from the threat of Xyacti invasion, sold a number of concept-armor devices which offered the Tesseracti some protection. 

The second expedition into Xyacti Victoria revealed what had happened. The darkness was broken only by faintly luminescent blue fog and the fluorescent alien witchlight that beamed from what used to be gaslamps; the bricks in the walls seemed to vibrate according to some unfelt frequency. A brief exploration into a record shop revealed rows of severed human heads, each encased in a weird sort of wax and still attempting to speak as Xyacti implants explored the neural pathways of their minds. The monorail had taken on the aspects of a living creature, sections of monorail track dipping down to gnaw entire stories off of buildings. 

The authorization for destruction by neutron bomb went through in record time. Once they'd been dropped, though, the followup expedition found out that nothing had happened, and lost a couple of bunnymen in the process. The neutrons bombs had simply been chewed up. 

Xyacti Victoria was finally subsumed into the Xyacti home dimension in 1994. However, interactions with the Xyacti have revealed that much of the Victorian Station's elements were integrated into the Xyacti home dimension. Xyacti invasions now frequently include Xyacti versions of Victorian Station-style architecture, clothing and music making their way into the invaded dimension. Apaprently, the Xyacti are under the impression that humanity's natural state is that of Victorian Station, and the dimensions that it invades are simply odd offshoots of the original genus. Given that the Xyacti are unable to tell the difference beteeen human life and the things that it produces, the Tesseracti have begun a systemic purge of anything that could be considered Victorian in their technology or architecture. So far, it's been pretty successful. They think.