Greyloch.

In the 22nd century, mankind has mastered the void. To do this, engineers on the now highly populous colonial edifice of Mars began work on an audacious project at the end of the 21st century which came to be known as a Relativity Gate.

The concepts of interstellar travel, propulsion systems for spacecraft, and the limits of technology had been a cumulative problem for humanity since the Space-Age of 21st Century Antiquity.
The mechanics of a Relativity Gate are bound by the sacrosanct laws of physics.
Powered by miniaturised fusion cascades (captive micro-dwarf stars.) The Gate collapses space time, essentially dragging two points together and holds a corridor open, allowing a spacecraft to pass into a far flung region of the galaxy wherever the Gate's co-ordinator aims the collapse point.
It is Mankind's Arc De Triomphe in the race to discover all there is, and where to colonise new worlds.

With the creation of the Rel-Gate, craft may be flung into pre-determined co-ordinates of the Milky Way instantaneously.
However, once the craft arrives, it is absolutely isolated in deep space without a means to return...
(Conventional propulsion drives would relegate centuries of travel between stellar systems.)

We humans have developed a simple means to combat this unfortunate side effect of our ambition. Load components, equipment, technicians, and materials aboard starships to construct Rel-Gates at their destination, process the return Gate to Co-ordinates for home, and establish an instantaneous link between planetary systems separated by millions of parsecs and thousands of years of conventionally powered travel otherwise.

The Relativity Gate Network serves as the best means to explore and inhabit the few regions of human occupation.

It is also hubris...

On February 16th 2180, an Explorer craft 'Pisces' sent into the Sirius constellation was lost with all hands. Distress calls were beamed back to Earth, but would take months to arrive at luminal speed. This prompted a second, Search and Recovery craft dispatched to ascertain the fate of Pisces and her crew.

The rescuers succeeded in constructing the Sirius-end Relativity Gate, then tracked the missing space craft to a small moon amongst 50, orbiting a gas giant. Designated Hol, the system became a surreal origin for terror and destruction stretching into collective societal awareness for decades after.
The moon coded : Hol-43 (later named the 'Piscean Cemetery') was found to be the site of ancient glory and calamity in equal measure.

The crew of the original mission aboard Pisces discovered the moon to be hollow, lined with millions of chambers, hollows, and architecture revealing significant autonomous intelligence. Deeper still, the core of Hol-43 held untold quantities of hushed figures in dormant caskets, any sign of animation suspended in frozen eternity clustered upon many hundreds of the surfaces comprising the moon's inner sanctum.

As is the nature of Man, awe, delight, curiosity and stupidity tempt him onward...

Hands scuffled with unknowable workings, minds tampered with technology baffling to comprehend, let alone operate.
It became inevitable then, that so far below surface level, and so deeply engrained in discovery, Man is his own lamb led to slaughter...

The beings awoke, bursting from disturbed slumber to tear petrified faces off of unsuspecting spectators, impaling those cornered by many times their number, or frightened fleeing, ambushed and sliced apart faster than decompression from a violently opened spacesuit could kill in Hol-43's sterile vacuum.

These were dubbed by the survivors of the second rescue mission, 'Holcroids'. Barely few, escaping the same fate as those who in blind desire for knowledge once crewed the Pisces.
An age-old question was that day answered...
We are not alone...

Of the returning survivors, their accounts dissected or conferred/compared with each others - named the Holcroids an imminent threat.
Here, so early in the nightmare brooding with hostility, a mandate was laid down, but without knowing how many the Holcroids numbered, their strongholds, their capabilities, military action would be vital if perilous...

The Colonial Military Sanction, commissioned the Integrated Regular Army with skirmish operations to determine the overall level of destruction Holcroid intelligence harbours.
Needless to say, the outcome was poor for the Army...

Holcroids are a symbiosis of machine and organism, able to disassemble and recombine mechanically to maintain more advanced shapes or constructs with various battlefield and non-battlefield roles. This has proven devastating to the Colonial Military Sanction - Fleet, and the Army. The side that adapts its tactics fastest undoubtedly wins the skirmish.
Strategically, Man understands nothing of Holcroid intent. The aliens are totally anathema to human life, and little if any data has been collected on them to serve humanity an advantage. To say that Holcroids are in possession of any kind of culture is not correct, it is simple mechanistic sophistication.

An extreme efficiency with individual and territorial defence, recombinant/disassembly technique, utility of advanced macro/micro cybernetics and strategic manoeuvring in space and planetary surfaces has the Human-Holcroid War surging with unimpeded momentum.
Other branches of human military efforts in the colonies and on Earth, have joined a mass mobilisation to secure the widespread Relativity Gate Network and those territories bound to them whilst prosecuting the Holcroid War on whatever front opens up.

Few systems under Holcroid control have been identified, and as yet, fewer human controlled worlds have suffered attack.
Though the initial first contact scenarios have met with dire consequence, a dangerous and little-understood enemy pushes against the glass dome of Man's achievements, it will shatter should we allow the weight to become untenable.

The Holcroids are lethal adversaries, their obscurity leaves a great scar of reflection on us with all of humanity hanging in the balance...