Clarence DeMarco was an Eastern European serial killer active in Victorian London. In 1893, he was apprehended there for murdering fourteen women and was sentenced to death. He was chosen by the Great Intelligence as a courier of information for the Doctor's associates, an encounter which put him into a traumatic state of mind.
The day before his execution, he was approached by the Whispermen and the coordinates to Trenzalore were impressed onto his memory. They claimed that if he cooperated with them, he would be spared his execution to live a good long life, except for troubled sleep. (HOMEVID: Clarence and the Whispermen)
In the end, his execution was postponed because he helped Madame Vastra by giving her information related to the Doctor, such as coordinates that led to Trenzalore and the comment, "The Doctor has a secret, you know. He has one he will take to the grave... and it is discovered." (TV: The Name of the Doctor)