A selection of Yogzarthu.
The Yogzarthu are a race of shapeshifters from outside our universe.
The etymology of the word 'Yogzarthu' is unknown, it is as close to the sound as Standard can render, and it is both singular and plural. The Yogzarthu language has so far been entirely untranslateable. Certainly no human can speak it, as it requires modular vocal apparatus.
Yogzarthu are a much older race than humans or Aquaans, having had an advanced civilisation since at least two and a half million years ago.
Little of Yogzarthu culture is known beyond that they're highly individualistic and their idea of social organisation is 'might is right'. Their names follow a [Name] the [Title] pattern, with the title being a plural, such as with Iahutta the Flock or Aradneth the Legion.
The Yogzarthu War was the second time the Yogzarthu had invaded this universe. The first time was over two million years ago when they had been accidentally brought through by the Qohathoth. An account of the Qohathoth-Yogzarthu War is written on the Dead Zone Monument. In short, the Qohathoth, lonely, unable to find sentient life in our galaxy, started reaching out through the dimensions in hopes of finding companionship. They found the Yogzarthu, who seemed friendly until they set about on a campaign of genocide to claim this universe for themselves. The Qohathoth chased them back into their universe and sealed the breach.
It was later discovered that the use of underspace causes signals to reach the Yogzarthu universe, and they can latch onto these and use them to guide their ships through. After the war, the Dead Zone was established - a thirty light-year buffer zone around the star system that marks the place where the Yogzarthu universe comes closest to ours.
Despite their biological skills, it was soon dicovered that humanity had several advantages over the Yogzarthu. One was that the Yogzarthu are not technically-inclined - most of their war technology was copied from humans. More importantly, they had zero hierarchical structure and no concept of cooperation, thus were completely disorganised and fought each other as fiercely as they fought humanity. A few of their ships might team up for a battle, but for the most part they struck randomly with no strategy.
The Mahi Expeditions
The original Mahi Expedition was headed by Kiace Mahi. He found what would later be called the Dead Zone Planet.
The Second Mahi Expedition was headed by Habynah Mahi in an attempt to clear her father's name. The trip into the Dead Zone was taken under conventional rocket power, with the crew in stasis. The Second Expedition returned with the records of the first, where it was found that Kiace Mahi and his crew attempted communication and proper first contact procedures with the aliens before the Yogzarthu slaughtered them and used their records to learn about humanity.
The original evolved appearance of the Yogzarthu is unknown, though assumed to be humanoid and somewhat reptilian, since they tend to favour such forms. It may only be that a synapsid humanoid body best suits their native environment.
As far as can be determined, Yogzarthu shapeshifting is a combination of the ability to rewrite their own DNA and a powerful regenerative factor. A dead Yogzarthu remains in the form that it died in rather than reverting to its 'natural' form.
They have the ability to copy DNA by ingesting or otherwise absorbing it.
It was feared that the Yogzarthu would take human form and attempt infiltration, but this never happened. Yogzarthu psychology prevented it in two ways: One, infiltration doesn't occur to shapeshifters who have only ever fought fellow shapeshifters - there's no point disguising oneself as someone when everyone can be anything. Two, the few times they took on human form, they were utterly unable to duplicate human mannerisms.
Yogzarthu can be detected in various ways. Sibyls can sense them at a distance. A detailed DNA test will reveal a type of under-DNA that allows a Yogzarthu to always be a Yogzarthu regardless of form, otherwise it would shapeshift once and get stuck. If a Yogzarthu attempts to take on the form of a specific person sight unseen, it is almost completely guarenteed to get it wrong, as it isn't just DNA but environmental factors that influence appearance.
One odd bit of fallout of the Yogzarthu War was that humans stopped bothering with DNA scanners for security and went back to more old-fashioned fingerprint scanners and passwords. The Yogzarthu could beat DNA scanners easily, but since fingerprints are influenced by environmental factors, Yogzarthu couldn't duplicate them correctly.
Given their shapeshifting abilities, it is nearly impossible to classify Yogzarthu reproduction. While they do collect and synthesise DNA, the synthesis is for their own benefit rather than for reproductive purposes proper.
For ease of description, Yogzarthu are generally referred to as parthenogenic. This was part of the danger they posed - with enough time and an adequate food supply, even one Yogzarthu could populate a world.