The Elders are Fiends of Chaos, they were never human but gained the ability to interact with the world in a way we recognize as intelligent from humanity. Their power rivals that of the Abidan Judges and just as the Abidan manipulate The Way the Elders are suspected of manipulation of Chaos.
Asylum is practically a holding cell to imprison the Elders. If they get out, they're scary. The Abidan originally intended to use it temporarily til the Elders fully corrupted it so that Ozriel could destroy it. Which wouldn't kill the Elders, but would drive them back into the void. So it was really just keeping them off the streets for a while. Then the Emperor came along, and humanity has prospered beyond what the Abidan ever expected. That tethers the world more strongly to the Way and strengthens the prison, weakening the Elders. So Asylum has held on much longer than anyone anticipated.
The Elders, don't want to be trapped there OR cast back into the void. They want to escape into other Iterations, which is exactly what the Abidan don't want. Back in the day, the Elders could have depopulated humanity. They didn't; they kept just enough alive to make sure the Abidan wouldn't obliterate the place while the Elders looked for a way out.
Asylum is a protected world to ensure the Elders don't escape due to Corruption of the iteration. This is also why the Emperor has not been offered Ascension though he clearly meets the power requirements. The planet is effectively sealed off.
The Elders each have a specific nature and each of the Elders usually act according to that nature. When they were free this usually meant that they stayed out of each other's way, but while they are trapped on Asylum, this can mean that they sometimes come into conflict with each other, such as with Othaghor and Kthanikahr.
The 7 Great Elders imprisoned on Asylum are :
- Nakothi, The Dead Mother
- Kelarac, The Soul Collector
- Ach’magut, The Overseer
- Urg’naut, The Creeping Shadow
- Tharlos, The Formless Legion
- Kthanikahr, The Worm Lord