The Compassionate were a species who existed during the Last Great Time War. The Time Lords feared that they would side with the Daleks so Rassilon trapped them in a rift at the heart of the planet Galen.
The Ninth Doctor later visited Galen and discovered that the Compassionate were influencing the people on the planet to kill. He used his old sonic screwdriver to seal the rift, trapping the Compassionate forever. (AUDIO: The Bleeding Heart)