Death's Champion was the title that the Eternal Death bestowed on those who agreed to become her champion.

In the Doctor's youth, he killed Torvic and felt immense guilt. When Death came to him at night, he made a deal so he could forget his crime in exchange for his friend, the Master, becoming her champion. The Master then left Gallifrey and spread horror and death wherever he went. (AUDIO: Master)

The Valeyard warned the Sixth Doctor that, in the coming conflict, Death would choose a champion to face Time's. (PROSE: Millennial Rites)

After the Seventh Doctor became Time's Champion, Mortimus agreed to become Death's Champion in an attempt to stop him. (PROSE: No Future)

The Seventh Doctor made a deal with Death to give the Master a new start in life as a good and honest man, John Smith, for ten years, feeling guilty for how he turned out. However, after the ten years, it was agreed he would have to kill the Master. When the ten years expired, Death sent the Doctor to Perfugium, where "John Smith" was living. Taking on the identity of John Smith's maid, Jade, Death tormented the Doctor, John Smith and his friends. This led to one of his friends, Victor Schaeffer, killing his own wife, and John's love, Jacqueline. When the Doctor couldn't go through with killing John, he realised his significance in creating the Master, and how he had turned him into Death's Champion, by making his original deal with Death.

He constructed a new deal with Death, so that he wouldn't have to kill John, but would instead have to kill an innocent person in the place of an assassin (who was just a disguise of Death again), but couldn't go through with that deal either. Death also made a deal with John Smith that to remain as John Smith, he had to decide whether he would make the Doctor Death's Champion, or if he would kill Victor so that Jacqueline would be reborn, but become the Master again. (AUDIO: Master) Ultimately, the Master had returned by the end of the Doctor's seventh life. (TV: Doctor Who)