The Ilk was a massive grey creature the size of an elephant. It was covered in eyes which varied in size and position. It also had several pseudopods.
The Ilk fed on strong emotions, though they couldn't digest disgust. They could also communicate telepathically, having a "deafeningly" powerful voice. They could also produce psychic energy to kill people, though only when properly focused. They produced psionic residue which smelled of tapioca and/or candyfloss. (AUDIO: The Forever Trap)
At some point an Ilk found itself in the Edifice. It reprogrammed the controls so that the building would teleport anything and everything it could, bringing more people to feed on. The Ilk could then devour their psychic energy. This continued for at least two years.
When the Tenth Doctor and Donna Noble were kidnapped by the Edifice, they searched for the person responsible, eventually finding the Ilk. The Doctor tried to shut down its controls, but the Ilk attacked him psychically. When Donna realised what the Ilk had done, she confronted it. Her negative emotions sickened the Ilk, so it tried to attack Donna instead. This gave the Doctor the chance to revert the controls of the Edifice, teleporting the Ilk somewhere without people to feed on. (AUDIO: The Forever Trap)