The Forerunners
Twenty million years ago, a highly technologically advanced global civilisation was was found over the entire planet. It arose, thrived and died in a geological eyeblink. It seems to have been dominated by a single intelligent species, but the move that established the Forerunners took no position as to whether their descendants are still found in some form or are extinct. These people are just called the Forerunners for the purpose of the move.
A few very old and very mad ghosts that no wizard, priest or shaman has been able to successfully communicate with and which seem to be closely tied to Forerunner sites may be associated with the Forerunners - or not.
Some thinkers have speculated that what would be left of humanity for future intelligent beings to find if we were to suddenly disappear would be virtually undetectable after a mere million years, let alone twenty. That is somewhat true, in this case, as well. But these beings left a deep impression on the planet, and it is possible for the world's modern denizens to deduce meaningful things about this important fact of the world's history. They may or may not have done so to some degree.
The Forerunners had an internal mineralised skeleton. They buried their dead, often in containers that were very good at sealing out the elements, and were so numerous at one time that they left plentiful fossils all over the planet in that 20 million year old stratum. Fossils of the Forerunners have even been inadvertently found through processes like weathering of cliffs, such that modern peoples the world over have stories to explain them.
The Forerunners built enormous structures, including out of materials that are not currently abundant on the planet. Some traces of these structures can be found preserved even twenty million years later. The exact circumstances needed to preserve Forerunner structures and technology in some form were rare, but the Forerunners were so ubiquitous that these traces are nevertheless not incredibly hard to find, especially if you're looking for them.
The Forerunners left a few, rare surviving inscriptions in several languages, although one particular language can be found practically anywhere on the planet and others seem to be more localised. The scripts themselves are markedly varied.
High aetheric activity is often associated with Forerunner sites. They appear to have pracised magic and highly advanced aetherics.
Impacts the Forerunners left on the world for those who know how to look:
The time of their appearance is associated with a rapid spike in carbon dioxide levels and an attendant spike in global temperatures.
A massive change in the distribution of plant and animal species is associated with the time they appear in the fossil record. Thousands of species abruptly disappear, and other species that were previously geographically isolated suddenly appear planet-wide in a geological eyeblink. As well, many species appear to radically change precisely when the Forerunners walked the planet, and some species first appear in the fossil record as if out of nowhere at this time. These sudden appearances continue to occur, albeit at a lesser frequency, in the time after the Forerunners disappeared.
An important Forerunner legacy is their massive landfills filled with their trash that they left behind, some of which have made it through geological time as literally fossilised garbage, yielding a wealth of information about the Forerunners.
There is also a thin layer of radioactive isotopes that appears in rock strata planetwide closely associated with the Forerunner period.
Because of this civilisation's adeptness with high technology and magic, it cannot be discounted that even after these forsaken eons, a few Forerunner sites, artifacts and inscriptions may still be very powerful and dangerous.
