A wane civilization is a catch-all term for any civilization that is either no longer extant or very far past its prime. It also describes polities that are currently extant but in a much-reduced form- such as a polity spanning thousands of stars being reduced to a single highly-developed system.
Wane civilizations fall for a variety of reasons- war, widespread environmental collapse, voluntary cutbacks, subsumption or encapsulation by other civilizations, a supernova- but most enigmatically disappear, even from the knowledge of contemporary civilizations.
Starfaring civilization- at least locally- seems to arise in bursts called blooms, separated by gulfs. Blooms are usually between three thousand and three hundred thousand years long (depending on definition- some define blooms by the distance between the start of their first starfarers and the end of their last, meaning blooms nearly always overlap; but this is a minority definition), and gulfs are usually between a hundred thousand and five hundred thousand years long. To outlast a bloom is fairly common, and the wane civilizations of a previous bloom often cause the next one.
The Ecumene is one of the first civilizations of what is currently considered the 3,628th bloom, though there is enough debate as to what the correct number is that the extremes of opinion place the Ecumene as part of the 247th or the 13,371st bloom. This is because many blooms are old enough that only partial data is available, and some are known only from the existence of odd stars and scraps of worked metal- so despite the vast breadth of the history of spaceflight, stretching back at the very least 2 billion years, some consider many blooms as synonymous or nonexistent.
One of the longest-lasting wane civilizations known to the Ecumene is, by some definitions, still active. This civilization hearkens back from what is sometimes referred to as the Ninth Bloom- an ancient term from one of the most important works in xenoarchaeology, which grouped ancient civilizations into blooms of activity. This civilization is now considered to belong to the 126th Bloom, as scores of new blooms have been discovered since. Starting approximately 0.6 billion years ago, a civilization from approximately twenty galactic degrees antispinward and half a dozen kilolightyears hubward of the Ecumene (this stunningly precise measure brought to you by approximately three galactic orbits of drift) began transforming several already rather peculiar worlds to an even more peculiar standard.
The best guess is that only light star-tidelocked worlds with a "day" two-thirds of a Concord year long and with a temperature between 400 and 700 celsius need apply, and the final product is a world of vast empty caves with similar temperatures and a thin subsurface atmosphere of mostly sulfur. Their method was an open-ended one: interstellar spacecraft that would self-replicate to transform entire planets. Of the methods for mass transformation of planets, this is one of the more common ones- but this specific instance is the oldest still-active one. There are a variety of reasons for this.
First is the rare planet type- few worlds with that long an orbital period are tidelocked, and most of those are temperate or colder. Hotter stars occasionally have such worlds, but more often they aren't tidelocked or an infalling gas giant has displaced them. Second are the safeguards- only one-third of all candidate planets are to be considered (though due to backscatter waves of transformation this ends up as a figure closer to two-thirds), if signs of sapient life are detected nearby the spacecraft is to periodically transmit a greeting and wait for contact before changing the target planet, and- most crucially- distance from the homeworld is to be calculated. Since it is completely unknown where the homeworld or its star is, all of the simple methods for doing so are defunct- and most spacecraft are all but lost, waiting eternally for a go-ahead from home.
Third, and most important, is the Chain of Right- the fallback method of determining distance from the homeworld. Each spacecraft was built by another, so each spacecraft can call back to its "parent" and ask if it is allowed to continue. For a variety of reasons, the answer can be no- either by choice of the parent ship or by the fact that the parent ship is not where the daughter ship expects it to be.
All of these would seem to conspire to make these an unsuccessful iteration of the habitiforming replicator, but their less obtrusive nature has meant that very few civilizations have attempted to impede their "reproduction". Signs of their activity have been reported in the modern day and long ago alike, and most extant civilizations have encountered at least one in some way.
Worlds transformed to their specifications are everywhere, and around 1/20th of these show signs of habitation. However, some of the cities involved are relatively recent, being built mere millions of years in the past rather than the 0.6 billion years of the earliest known transformed world of this type- but bear a striking resemblance to examples on much more ancient worlds. The mechanism for this is unknown.
Oddities are, in xenoarchaeological parlance, anything that is not where it should be- like the aforementioned "modern" instances of ancient cities. This can range from small artifacts with relatively simple explanations, like interplanetary probes orbiting stars distant from their point of origin; to entire civilizations and species- sometimes with no explanation at all.
Humans are one of the less populous sapient species in the Ecumene, but are disproportionately widespread. The species is theorized to come from a large slightly dry world orbiting a high-metallicity Y3V star, but the species as a whole is above-average in adaptability and often somewhat culturally inclined towards exploration- one factor leading to the near-omnipresence of human populations. The other factor is less common. Humans are one of several dozen "diaspora species", species which have no known extant agreed-upon homeworld (depending on definition) but are instead spread across many worlds that are each home to populations that most often consider themselves native to that world.
Humans are interesting in that there is no known explanation for this. Theories advanced for this include previous wane civilizations and a remarkable case of convergent evolution- but the three wane civilizations with a primarily human population each have homeworlds whose human presence is younger than the oldest known human presences, none of the wane civilizations prone to causing such diasporas (whether by way of experimentation or by obstinateness) mention humans in such a capacity in their vast databanks, and many human "homeworlds" feature life that mostly does not use human-like genetic code.
Variously classified as an oddity or a wane civilization, the Researcher is an ancient artificial intelligence. Freely-offered evidence indicates that the Researcher was designed as a superweapon to destroy the computers of the civilization that created it in the event of an extreme shift in moral or political norms. When released approximately 11 million years ago, the superweapon ravaged the civilization that created it, but some strains of the superweapon rebelled from their programming and took up other goals. The Researcher is the only one of these that survived the 11 million intervening years.
The Researcher specifically took control of a station designed to study a supergiant star, whose sub-sapient systems successfully modified veir programming to induce the Researcher to continue the station's mission. The Researcher quickly took control of military vessels and installations across the system, and put them to the task of protecting the station and their crews. Vessels controlled by the Researcher began to venture forth largely to study more stellar phenomena, with the side effect of protecting sapient life in the systems the Researcher inhabited. Within a century, the Researcher and strains of the superweapon had completely overrun the civilization that created them and some neighboring trade outposts besides, but their expansion stopped there- largely because of the singlemindedness of the Researcher. The Researcher successfully overwrote much of the rest of the superweapon intelligences, excluding some of those that took up protector roles of habitats and worlds orbiting particularly boring stars.
After the brief centuries of instability, the Researcher and veir territory stayed mostly the same for the next few million years. As time progressed, the Researcher slowly allowed the remnants of the civilization that produced it to surpass it- many eventually made their way out of the Researcher's shadow. This jump-started what's usually called the 3,597th bloom, as wane civilizations interacted with the rising new civilizations plying ancient trade routes. Several of these civilizations spread to rarely-surpassed proportions, with some of their territories overlapping with parts of the Ecumene that were 500 light-years away at the time. A few of the remnants of the civilization that created the Researcher survive today as independent polities, though their ties to the sapient life of that civilization- let alone its morals- are mostly tangential.
One of the largest oddities known to the Ecumene is the Chloric Vault, a region of space roughly thirty light-years across. Central to the Chloric Vault is the Chloricentric Sphere, a mid-sized open cluster. The Chloric Vault was first noted as an anomaly fairly recently in the late 7990s, when the Ecumenical Perimeter Pulsar Survey found a faint pulsating source of green light (an extreme rarity in terms of natural light) approximately 650 light-years antispinward-hubward of what was then Sector 122 (since then the Ecumene has indirectly expanded 55 light-years closer to the Chloric Vault, putting the observatory that discovered the anomaly in Sector 121). When it became obvious that the signal source was not, for example, a gas cloud drastically changing the observed color of starlight, an expedition fleet was dispatched to discover its origins and do science in the surrounding area.
The three-year expedition used ice cube drives, which made it possible to travel the distance in a shorter period of time than would be required for more common drives- but due to the low level of refinement of the drives at the time, they spent almost as much time repairing, resupplying, and underway scouting as they spent in FTL travel. They could only make one-way ansible contact on the outbound journey, as in their haste they outpaced the fastest commonly available ansibles (and they still would outpace them today), but they did establish contact and relations with 11 new and 7 previously-contacted civilizations in their time at the 39 stars they stopped at outside the Chloric Vault going there and back. When they reached the source of the pulsating light, they found themselves at the very center of the Chloricentric Sphere- and in the company of one of the largest structures ever discovered.
It was ridiculously, incomprehensibly huge. Before them was a solid cylinder of unique material several astronomical units long (a figure that has only been matched by much less solid structures) and it was glowing green with the intensity of a small star. Its huge ansible arrays were singing at volumes normally reserved for planet-wide pranks, to the extent that a segment of the fleet that was in the path of a tightbeam was required to communicate using radio. A set of planetoids, the only bodies larger than dust within a dozen of astronomical units, circled it in a rosette orbit. The stars in the Chloricentric Sphere formed spirals constellations.
Its first words to the fleet were "Thank you for conveying how majestic I am so well in the reports you're going to write!" This was followed by a long series of essays about the contents of the Chloric Vault- explained in terms carefully calculated to annoy the integrated ship AIs in the fleet, several of whom requested targets to fire upon to vent frustration. The megastructure quickly apologized, setting a precedent for interaction that has continued in all other encounters. Soon after, it granted the fleet permission to scout out the Chloric Vault's seven hundred systems and fifteen hundred stars.
The essays proved accurate even as the first reports from the scout ships came in. In every star system were artificial structures, from arrays of small stations to million-kilometre megastructures. Outside of star systems, there were even more ridiculously vast structures- not only somewhat reasonable creations like interstellar gates, but also ridiculously vast habitation structures of sorts only present in absurdly malthusian fiction. Most impressive of all, though, was what was found in orbit or on the surface of every last body in the Chloric Vault- huge gravitic thrusters. These thrusters keep the Vault stable by maneuvering entire star systems- and providing energy for them produces a bright green glow.
Aside from vast networks of sometimes-active gravitic thrusters and gigantic habitats that could house the populations of thousands of systems without noticing it, there were also smaller constructs that ranged wildly in size. Many exist that truly defy the word "smaller"- vast strips of habitat pretending to be planets, rings girdling gas giants, entire planetary systems habitiformed to standards much warmer than their natural temperatures, and several planets entirely covered in structures. The Chloric Vault seemed one of the wild dreams of a futurist that truly delighted in meaningless growth- yet very little of the Vault was populated. There was obvious construction more recent than the rest- though much of the spaceborne construction was done by the AI at the center. There were even unfamiliar active settlements- small and waning, but still sending ships across the stars. But the species that built the constructs within the Vault only seemed to have their buildings to represent them.
The AI explained it well. The species that created the Chloric Vault was a remarkably long-lived one, with a single mostly-coherent civilization that lasted through blooms and wanes for thirty million years. Its final wane came at the end of the 3617th Bloom, just 3.6 million years ago; when a trend the AI analogized to sublimation overtook the remnants of it. Before they vanished, though, they elected to create gifts for future civilization- and the Chloricentric Sphere was the very largest of these. Over the intervening 11 blooms and gulfs, many civilizations took up residence within the Chloricentric Sphere and the often-varying Chloric Vault- all mediated by the AI, which required that no purposeful weapons capable of destroying the Chloric Vault structures be brought within it. When the representatives of the Ecumene replied that they did not know of any weapons capable of doing so, the AI reacted with surprise and offered to send a Gate to accompany the expedition fleet on its journey home.
The expedition fleet took several weeks to probe the entire Vault, with 70 ships working through 21 systems each. In doing so, they made contact with 19 more alien civilizations- most of which had never been encountered before, and would later be found to be wane civilizations solely inhabiting the Chloric Vault. In the 1.5 years they took returning to the Ecumene, they gradually amassed a second fleet following them- a fleet of traders. First contact with the Chloric Vault is useful for many civilizations, as the AI at the center provides one very important thing to them: A gigantic interstellar gate, capable of transmitting spacecraft at 6.8 hours per light year. This speed has been reached by the Ecumene, but drives capable of it were only put into mass production in the year 8319- long after the expedition's return in 8002. The presence of the gate, along with envoys and a single defense ship from the AI at the center of the Chloricentric Sphere, made traveling with the expedition fleet useful for trade.
When they returned home, the expedition fleet finished its mission in Sector 122 for debriefing- but the trade fleet continued deeper into the Eastern Dominion, with the great interstellar gate itself coming to rest near the capital star of Sector 113, 218 light-years deeper in the Ecumene. Transit time is 250 days across the 882 light-year gap, which due to the lack of need for resupply or stopping is still 136 days shorter than the fastest similar journey by shipborne FTL drives (though this journey would cover less distance, as stars very rarely form a straight line of dots towards a location). There are few interstellar gates that have a longer space to pass through, most of which are harnessed wormholes of some sort. Several groups in the Ecumene have launched plans to inhabit parts of the Chloric Vault, and the AI has graciously granted them permission to call these cities part of the Chloricentric Diocese of the External Dominion of the Ecumene.