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)
Biology
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)
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)
Culture
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) Dorothea Ames understood that Quill mothers died in childbirth, and so their children feasted on the corpse in the nest. (TV: The Metaphysical Engine, or What Quill Did)
Prior to their war with the Rhodians, many Quill worshipped the Quill Goddess. It was believed that she rose from the underworld and burst out of the first Quill nest. Most, however, lost their faith in her when the war went in favour of the Rhodians. (TV: The Metaphysical Engine, or What Quill Did)
Charlie, Prince of Rhodia, claimed that all Quill were heartless. (TV: Brave-ish Heart)
History
According to Charlie, Prince of Rhodia, they mismanaged their economy, and grew resentful of the Rhodians' success. Miss Quill added, though, that the Rhodians ate up all of the planet's resources, leaving them with little to work with. The Rhodians did their best to help, but in the process made them so dependent that they could never recover.
The two peoples were eventually at war with each other. The Rhodians won. The punishment they received was considered cruel by Miss Quill, but Charlie found it just: they 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)