When the Sun was a "young star", a scientist had invented the Umbra, a fabric fashioned into a cloak which drained guilt and regret. He claimed that his name was "long dead, [and] drowned in shame", and that he was "only the Fool now. The Betrayer."
Eventually, the Umbra evolved and gained intelligence and began devouring people's minds and souls and spread across his planet. The scientist lured them into the Sun using what the Twelfth Doctor guessed was "most likely a trans-warp tunnel, using the Sun as a power source", and the Umbra were trapped in the Sun's chromosphere. Alice Dubrovnik, realising that the scientist's guilt over knowing he saw his world die, would have been "off the scale", the scientist would have used his own immense guilt and regret to lure the Umbra together.
The scientist sent a looped signal which bounced around the chromosphere containing his final statement as a warning in case the Umbra could survive being sent into the Sun. Sandy Tanaka guessed the signal had been running for millions of years. (COMIC: The Eye of Torment)