Eknodine

The Eknodines were a "proud, ancient race" of aliens, according to the Eleventh Doctor. A group of Eknodines appeared within a dream world created by the Dream Lord, in which they were depicted possessing the elderly men and women of Upper Leadworth.

Biology
The Eknodines appeared as aliens that could live inside human hosts, extending their lifespan and increasing their physical strength. Though most of the body of the Eknodine was hidden inside the human host, a green eyestalk could be extended from the mouth, which would spray out a green cloud when attacking. This cloud could turn humans into dust. They also had thin tendrils that protruded from the eyestalks. (TV: Amy's Choice)

History
The Eleventh Doctor was familiar with the Eknodines, describing them as a "proud, ancient race", and apparently not originally inclined towards violence.

The Doctor, Amy Pond and Rory Williams encountered Eknodines in a dream world version of Leadworth created by the Dream Lord. The Doctor was bemused when he learned Eknodines had possessed the elderly inhabitants of Upper Leadworth and become violent; the Doctor chastised the creatures, telling them they ought to know better.

The Eknodines responded by claiming they were driven away from their home planet by "upstart neighbours"; they came to Earth, where they possessed the elderly inhabitants of Upper Leadworth's retirement home to stay inconspicuous, extending the lifespan of their hosts in the process. Angered at their loss, they decided to attack "others", such as humans. The Doctor remarked that the Eknodines' story "[made] sense, I suppose", acknowledging that the story had a chance of being real, rather than being a facet of the dream world, though he seemed unconvinced of either possibility.

The Eknodines later chased the trio through the town, with the trio having to fend off the creatures while occasionally falling asleep, which would cause their consciousness to flip back and forth between a parallel dream set inside a version of the Doctor's TARDIS. When "awake" within the Leadworth dream, the Doctor used a van to evacuate the town and get the non-possessed civilians to safety.

The Eknodines ultimately cornered the trio in Amy and Rory's house, attempting to gain entry. One Eknodine, in the guise of Mrs. Poggit, climbed to the top floor window and killed Rory, turning him into dust. The Doctor grabbed a lamp and used it to shove the Eknodine off the roof, neutralising the threat to himself and Amy.

After careful deliberation, Amy and the Doctor decided to kill themselves, hoping that the world of Upper Leadworth was the dream, and that they would reawaken with the "real" world of the Doctor's TARDIS. They used the van to run a couple of Eknodines over, before crashing into the house and "dying". Moments later, the trio apparently awoke within the TARDIS, though the Doctor later realised that both settings were dreams and set the TARDIS to self-destruct, which allowed them to finally wake up and break free from the Dream Lord's manipulations.

Since Upper Leadworth and all its contents were proved to be imaginary, everything that the imaginary Eknodines claimed about their turn to violence was probably false and did not hold true outside of the dream world. (TV: Amy's Choice)