The Sequence was first encountered in the Serene Sea when the Starmoth Initiative explored this strange expanse of stellar remnants, only to be promptly driven back to charted space by unknown assailants.

1 - The Mystery Ships of the Serene Sea

For a long time, the Sequence was only known through its ships, and these ships are terrifying. The efficiency of their engines makes them almost akin to reactionless drives, to the point some scientists are wondering if they are not outright breaking the known laws of thermodynamics. They seem to be using some kind of inertia dampening technology that enables them to perform seemingly impossible manoeuvers. Their weaponry relies on relativistic particle beams that are far more effective than any armament our ships can sport. They can manage heat to the point of rendering their ships virtually undetectable. The few times we managed to get a glimpse of Sequence ships their appearance had nothing in common with human designs: their lines were hard to define, their shapes blending in with the void, barely visible against background cosmic radiation.

Sequence ships, however, have a single, glaring weakness: they cannot travel faster than light. 

2 - Only Known in War

We've been in contact with the Sequence for eighty years, and these eight decades have been spent at war. A long, gruelling, yet low-intensity war concentrated around the moving frontlines of the Serene Sea. A war waged between ghost Sequence ships and Algorab, a non-governmental agency equipped with hastily armed exploration ships. A war with elusive targets and battles that don't seem to be really happening. By all accounts, a war that makes no sense, for a very simple reason: the Sequence has stopped existing as an actual civilisation about twenty thousand years ago.

3 - The Empire That Was

When the war erupted in the Serene Sea, the Starmoth Initiative naturally tried to locate the homeworlds of the Sequence, if only to find someone to talk to. Their search returned nothing but ruins. They only found vast ruined ringworlds interfering with FTL translations, barren planets orbiting dead stars, deep space stations hanging in the void like empty, icy needles. Sequence ships themselves look more like barely functioning wrecks than operational vessels. Their entire world seems to be made of ruins.

The recent rim-bound Laniakea expedition cast more light on the Sequence. First, it showed that the Serene Sea was only a fraction of the Sequence, with their ancient empire extending to the entirety of the outer arms, covering a non-negligible fraction of the Milky Way. In a very intriguing way, this ruined empire seemed to have developed following an isotropic pattern, spreading from star to star at 95 per cent of the speed of light across several million stars. It is estimated that the Sequence took between ten to twenty million years to build its interstellar empire, one star at a time, creating a vast network of colonies out of several thousand species, all of which have now disappeared. It seems that, at its apex, the Sequence could engineer the very stars themselves, carving ringworlds and Dyson spheres in their systems.

The reason why this empire fell is yet to be discovered - perhaps it is simply that civilisations have an expiry date.

4 - The Dark Tide

As the Sequence dissolved in time it only left ruins and ghost ships behind - the fact that these remnants are enough to be a real threat in the Serene Sea gives a staggeringly humbling insight into their ancient power. One thing is certain: the Sequence is wasting its last known resources fighting us in the Serene Sea. Their ships appear more battered and weaker with each passing year and the great ringworlds themselves are slowly falling apart, their self-repair systems hijacked by derelict military brains to manufacture combat fleets, often cobbled together and with very little, if any, combat value. Even though the sheer technological advancement of the Sequence makes them a formidable opponent in a straight fight, our ability to travel faster than light gives us such a massive strategic advantage that it is only a matter of time before the Sequence runs out of means to keep fighting.

But a lingering question remains - why? At first we assumed that Sequence ships were just mindless drones with a highly developed territorial instinct, defending the empty husks of a long-forgotten civilisation. However, the Laniakea expedition revealed that what we had taken for drones were in fact fully sapient creatures, probably the closest we'll ever get to a real, alive Sequencer. Their actions were the product of conscious thought, not idle reflexes.

Then, at the heart of a derelict Dyson sphere in the Serene Sea, a group of linguists managed to exchange a few garbled words with a dying Sequence interceptor. This is how the Sequence describes us: the dark tide that will engulf the stars.

To them, we are unthinkable creatures travelling faster than light, taking mere decades to achieve what took them millions of years, strange humanoids using an eldritch device capable of bending the very structure of time and space. In a word - monsters.