Very little is known about these creatures. One day they ambushed the Xyloxi in the 4th dimension, seemingly appearing out of nowhere. The Xyloxi are sure the Mith'ri'aeil originate from a dimension higher than the 4th.

Mith'ri'aeil are shapeless blobs, made of a purple substance that seems to be able to burn or eat its way through even the strongest armors. The Mith'ri'aeil possess no technology whatsoever, yet they are capable of travelling through different dimensions with ease, contributing to their unpredictable nature.

These creatures are naturally drawn to chaos, often with little regard for their own safety. They are quite vulnerable to energy based weaponry and while their own offensive capabilties, limited to detaching and launching small amounts of the purple substance they're made of, are quite dangerous, they lack range.

It's generally assumed that Mith'ri'aeil regard anything as food, as they tend to absorb any form of matter. This usually results in the specimen growing and eventually splitting in two smaller specimens, each going their destructive ways.
The behaviour of most of these creatures hints that they're driven by a primal, instictive hunger, yet at the same time they possess no other instictive urges or protections, essentially making them stupid. It's common for a single specimen to come upon an area with food (any kind of matter will do), eat everything in sight, and they remain inactive as it doesn't posses the intelligence to reason that more food might exist in other locations. If no external stimuli cause the specimen to move, it will simply remain in that place until it perishes.
During feeding, whatever they absorb basically becomes indistinguisable from themselves. This process thus turns matter into the material Mith'ri'aeil are made of. While the creatures have no traditional organs, relative complex structures exist within themselves that serve to make them aware of their surroundings, process stimuli, or allow them to move. As a result, a Mith'ri'aeil creature can become more intelligent when feeding.

There have been numerous cases where the Mith'ri'aeil absorb both creatures or ships, not to dissolve or digest them, but to use their strengths to boost their own. Especially Caes'cix seem to be targeted for this, resulting many forms and shapes of Mith'ri'aeil-Caes'cix hybrids, ranging from infected Caes'cix that somehow still manage to retain control over themselves to Mith'ri'aeil with many functioning Caes'cix talons.

When the Mith'ri'aeil first arrived in the Xyth galaxy, following Xyloxi that had fled the 4th dimensions, they quickly ended up in the regions, in the north-eastern parts of the galaxy, where the Caes'cix-Rivi'i war raged furiously. These regions were also nearby human occupied space. Encounters with the humans were few, but the most devastating one resulted in the loss of the first human supercarrier, Troy. The ship was later found in an abandoned Rivi'i base, partially absorbed and turned into an extension of a monstrous Mith'ri'aeil specimen, guarding an even greater monster.

Generally, Mith'ri'aeil operate alone, but there have been a few cases where large hordes of the creatures operated together to achieve a common goal. Who the leading force behind this is, remains unknown as well as how Mith'ri'aeil communicate with each other.

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