Aeternum is by far the oldest and most temporally anomalous object in the universe. Surrounded by a bubble of severely sped up time, it is approximately 101,500 years old, far older than the universe. Because of the time differential between Aeternum's sphere of influence and the rest of the universe, it is almost completely impossible for a spacecraft to enter it, because the first atom could orbit the object billions of times before the next even entered, resulting in any probes sent towards it to be shredded into fundamental particles.
Aeternum originally formed as a perfectly normal G-class star in the reaches of the Florathel Galaxy. What planets it might have had are unknown, but it probably had a civilization. Nothing particularly interesting happened to it until the Lareas Alliance decided to test its time warp technology on this star system.
The formation of the time-warp bubble permanently sealed off the system from the rest of the universe, and from the outside the star appeared to expand into a red giant and promptly collapse into a white dwarf after just three seconds. But none would witness this event, for in causing it, all in the Lareas Alliance were reduced to naught but cosmic ashes as a bubble of some form of lethal radiation expanded outwards at the speed of light. Only a minute later, the white dwarf had cooled into a black dwarf.
Over the intervening period, quantum processes caused every atom on the object to fuse or decay to iron, over periods so unimaginably long that the number of years required for this to occur wouldn't fit on a page. None know what happened to the civilization which existed in the system, but long-range scans have detected something very large around the Iron Star. Hypotheses for what this is include a matrioshka brain, defunct dyson swarm, or something even more bizarre. As of today, no one can figure out a way to penetrate the time bubble, so no one knows for sure what it is.
Under the current understanding of physics, Aeternum should have long since decayed into background radiation via proton decay. However, some believe that the timeline over which proton decay happens remains tied to the time of the universe as a whole, instead of that within the time bubble. This would explain the continuing existence of the object, but there is no way to confirm this.
Not much is known about Aeternum's properties, as it is impossible to view it up close. However, it is known to be almost completely composed of iron and is approximately the size of Carlea. It is not lit by anything, but beams of radar shone from outside the time bubble show the existence of a mottled surface covered in pits and mounds. Aeternum is surrounded by a very large object or swarm of objects, presumably some form of matrioshka brain created by the former inhabitants of the system. It is believed that any planets in orbit around the star were either disassembled or fell into the star via gravitational wave energy dissipation.
Long-range scans of Aeternum show a large structure surrounding the object. It is believed that this structure is some form of large matrioshka brain similar to the ones found in the Iriat Nebula. However, the light emitted from Aeternum itself is utterly feeble, and there's probably more light from the outside universe. The tiny amount of energy being fed into it means that computing must naturally slow down by a huge amount. Potentially, any beings uploaded to this computer would be running millions of times slower than space within the time bubble.
Other hypotheses include the existence of a large debris disk, which has been disproven by the relatively short timescales it would require to dissipate, as well as a system of planets, which has been disproven by the fact that the estimated age of Aeternum would require that any orbiting objects would emit enough gravitational waves enough to decay into the object.