click on images to enlarge
The Graske were small compared to Humans and had pebbly brown skin and three tentacle-like features on their heads. (Attack of the Graske) They had difficulty speaking other languages, having odd grammar and curt sentences. (Whatever Happened to Sarah Jane?, Music of the Spheres)
The Graske were a time-active species and could travel by different methods. Individuals could travel through time with ease, using a device on their belt to vanish in a distinctive blue glow. The Graske employed by the Trickster also had a gun which he could use to take people out of time and put them into Limbo. It could also reverse the process, putting them back in time and space and restoring their timeline. (Whatever Happened to Sarah Jane?)
When invading a planet, replaced the inhabitants of the planet with changelings. This process was quick, with the Graske replacing the individual with the changeling in seconds. The originals were then stored on Griffoth in stasis chambers in order to maintain the copy. The changeling could be identified by its glowing green eyes. (Attack of the Graske)
Russell T Davies’ original concept for the Shadow Proclamation scene in The Stolen Earth included an appearance by a Graske, but his plans to feature a “rogues gallery” of alien races in the scene fell through; ultimately the Whatever Happened to Sarah Jane? Judoon were the only returning race featured. Strangely, the Graske in the sketch is depicted as wearing a kilt and with fish-like feet, something it is never shown as on TV.
A Graske appeared along with Androvax and Kudlak in a promotional trailer for Series 4, the three foes having seemingly made an alliance. Androvax announced that the Earth would be theirs in 12 hours and the trio proceeded to laugh sinsterly. They were interrupted by Sarah Jane, who revealed them to be Clyde, Luke, and Rani respectively, using some sort of holographic device to pose as their old enemies. Androvax would be the only one of those three foes to return, unless you count the appearance of the Groske in Death of the Doctor.