The Quill were a humanoid species on Rhodia. They lived on the smallest of the southern continents, (TV: For Tonight We Might Die) and had a long history of combat and civil war with the other species on Rhodia, the Rhodians. (TV: Co-Owner of a Lonely Heart)
This could do with a physical description of the Quill Goddess from The Metaphysical Engine, as she is all we have to go by as to how the Quill truly look.
The one Quill known to have visited Earth, Miss Quill, was outwardly indistinguishable from humans, (TV: For Tonight We Might Die) but this was only because her true form was disguised. She claimed her true appearance could make humans scream. (PROSE: Joyride)
They had a more alien appearance originally, although it was not clear whether one form shown was their true form, or simply one imagined by April MacLean when she learned of Miss Quill's origin. In that form, the Quill, appropriately for their name, sported long quills or spines on their bodies, which sprouted from at least their heads and shoulders. Their skin was primarily reddish brown, with black, zigzagging stripes, and pale white on the lower part of the face. (TV: For Tonight We Might Die)
Quill would typically give birth to a litter of young (TV: The Metaphysical Engine, or What Quill Did), but this was not strictly true when it came to sexual encounters with other species. When Andrea Quill had sex with a Lorr called Ballon, the resulting pregnancy was only that of a single Quill. (TV: The Lost)
Quill had two wombs within them, and at some stage in pregnancy, the children would move to the second womb. (TV: The Lost) In an early stage of Quill development, Quill would shed their quills while still in the Quill nest. They would also consume their mothers, though Miss Quill noted that by her time this was medically unnecessary, and no more than a followed tradition. (TV: The Metaphysical Engine, or What Quill Did)
Miss Quill told a Lankin recreation of Orla'ath that Quills did not have souls. Orla'ath said that they did, and also said the Quills believed they died with them. According to Orla'ath, "all Quill sisters" would try to kill each other in the nest, to test each other's strength. (TV: Nightvisiting) Miss Quill told Dorothea Ames that Quill mothers died in childbirth. Ames understood that their children thus feasted on the corpse in the nest. (TV: The Metaphysical Engine, or What Quill Did)
The Quill worshipped the Quill Goddess. It was believed that she rose from the underworld and burst out of the first Quill nest. Most lost their faith in her, however, when, as Miss Quill explained, a thousand years before Quill's time, they realised that the only thing protecting the Quill was the Quill themselves. (TV: The Metaphysical Engine, or What Quill Did)
Andra'ath, of the Quill, was aware of Gallifrey, the Time Lords, and the Last Great Time War against the Daleks in contrast to Charlie, who was unfamiliar with the Daleks until he encountered one on Earth following Rhodia's civil war. (AUDIO: In Remembrance) She was also more aware than Charlie was of the poverty and destruction caused to her people by said civil war. (AUDIO: Mock)
According to Charlie, Prince of Rhodia, they mismanaged their economy, and grew resentful of the Rhodians' success. Andra'ath added, though, that the Rhodians ate up all of the planet's resources, leaving them with little to work with.
Charlie stated his belief that the Rhodians did their best to help, but Andra'ath argued that in doing so, the Rhodians had made the Quill dependent on them and that they could never be able to properly recover. Later, Andra'ath would be able to show Charlie how the Rhodians had put the Quill into a state of poverty.
Regardless of either species' perception, the two were eventually at war with each other. The Rhodians won and Andra'ath, along with other key members of the Quill side, were punished in a particular way: the Rhodians released a creature into their brains called an arn, which, once it dug itself in, would telepathically link the host to a particular Rhodian, and force them to become their effective slave.
If I use a weapon other than my own hands, the arn kills me, yet if I fail to protect you, I am also killed. If I try to remove the arn, dead again. Slavery!
While they were busy fighting amongst themselves, the Shadow Kin arrived, and killed everyone, Quill and Rhodian alike. All but one of the Quill were destroyed by the Shadow Kin: a female known as Miss Quill once brought by the Twelfth Doctor to Coal Hill Academy. (TV: For Tonight We Might Die)